How To Diagnose A Soft Brake Pedal

[@gregreed2713]

As an ex-TRW braking engineer knowing the guys who's name is on the drawings for that particular MC, I would say - nice job! You're right about the brake booster - any fault there is going to make a hard pedal, not soft pedal. The thing I missed in the discussion is about the dual circuits of the master cylinder. There are two independent circuits. Two chambers in the master cylinder. If I remember correctly for that model, it is a front/rear split. If the front circuit has failed, the pedal will feel almost like it goes to the floor. But if the had the car on the hoist, you would feel that the rears are locked. If the rear circuit fails, the longer travel would be less obvious - because the rear calipers are smaller, consume less fluid for a certain pressure. When you have an internal leak in the MC, it would be normal to have a soft pedal with slow apply, and hard pedal with fast apply. Dynamic effect - the fluid can't escape through the leak path quick enough, so you still build pressure. And yes - we designed those with circlips missing the holes on purpose - if the seals have worn, likely the bore has too. And it's just not worth messing with a rebuild to replace the piston center valve (I know, start the hate mail!)

[@popflorin-stefan5629]

Hi. Sorry for interrupting but I have a question about the ABS unit. What will happen if A valve is stuck open? Then the same sensation will be like a soft brake pedal because fluide will escape through that valve. Also when a wheel speed sensor is bad, the car thinks that the wheel is keeping blocking and the ABS releases the pressure and the pedal feels soft during it. I ask because I have an e91 and my car stops in waves like the road it's uneven.

[@zesiot6221]

@@popflorin-stefan5629 TRW PML 364 brake master cylinder for E39 TRW will work well in the e39 ?

[@johnathonmcalister6645]

Having done brake master cylinders on multiple old (pre-fuel injection) vehicles, I know that some of the gaskets, O-rings, and other rubber parts eventually wear out. At those times, brake master cylinder rebuild kits were easy to come by, and an evening's worth of work doing the rebuild would make the brakes feel like new. The "give away" symptom for me was a brake pedal that worked, but as I kept pressure on it, the pedal would slowly sink to the floor. Over a short time period (say a week or two), the pedal would sink faster and faster - until even the initial press on the pedal gave no braking, unless I pumped the brake ahead of time. I believe this was caused by fluid flowing around the internal seals of the master cylinder as they wore out - and the more wear, the easier the fluid could flow around them, until they offered no resistance to the fluid. Replacing the worn out parts restored the functionality, without the need to replace the entire master cylinder. I don't know if these rebuild kits are still being sold - would be a great FCP Euro DIY video if they are, as it's an easy project.

[@dazeboy79]

Thank You for doing a diagnosis video for these cars! I appreciate you taking the time to teach as a professional because braking is so important when driving! Great lesson on braking systems!! 5 Stars!!⭐⭐⭐⭐⭐

[@cuzimadapapushkian799]

Exactly what I was looking for. I'm trying to resolve a spongy pedal on our X164, and this issue is pretty much the same. I had a garage do the brakes and found they were soft. Took it back twice asking them if there was a leak anywhere, and to bleed the system. The first time, they changed a hard line from the ABS to the driver's side, but after that they looked at me as if I were from outer space. So, when I found a weekend with some time on my hands, I powerbled the system, getting rid of all the old brake fluid, but the brakes were still spongy. No visible leaks either. So now I'm in the process of replacing the master cylinder. Gareth's logical diagnosis makes me think this will solve the problem once and for all.

[@danny_playswithcars]

Make sure you also look into the recall Mercedes has issued for the X164 and V251 brake boosters! You may be entitled to a dealer fix, and it's a high-priority one.

[@cuzimadapapushkian799]

@@danny_playswithcars Thanks for letting me know about that. I thought I saw something about that online. But Mercedes didn't send me a letter. I'm in Canada so maybe it's just the US? I'll look into it further. Thanks

[@danny_playswithcars]

@@cuzimadapapushkian799 Yes, it is an open recall for US market specifically though I'm not sure about up North. I'd definitely recommend contacting your local dealer to give it a shot and verify whether or not you may be covered. The issue stems from corrosion around the booster, so it's possible you're experiencing a similar failure to the cars seeing issues down here. For reference and if it's helpful, our NHTSA ID in the US is "22V315" and may have helpful info for you. Regardless of the cause, best of luck tracking down your issue!

[@fernandm3208]

@@danny_playswithcars Thanks so much for all that info Danny. I really appreciate it.

[@askbascombe]

Remove the plastic panel forward of the left rear tire. Give that hard line a good inspection. The line on my 164 popped in that spot, pedal went right to the floor.

[@akashjemini2666]

This is a very helpful video. I have a similar issue with my brake. I did the diagnostic following your video and noticed brake fluid lead at the bleeder screw. The leak is very little that it wont even make drip even after i left it to leak for couple of days. I replaced with a new screw but still saw leak. Then I applied grease but that also did not work or fixed the brake. What do I do to fix the leak.

[@donaldarrabito6899]

Looking good my friend. Awesome info as always…

[@ronald5728]

Excellent video. Good analysis and investigation. Seems to be the same problem with my vehicle.

[@humanresources2138]

Wow, he lost a lot of weight. Good for him.

[@TomcatSTL]

yeah. unless Gareth is sick. I thought he looked healthy at his old weight.

[@ckegman6103]

great job and looking great dude.

[@fcpeuro]

Thank you kindly.

[@DirtBikeSessions]

Great video!

[@kuldipdhak7972]

Nice explanation

[@TylerShepherd-t1d]

Thanks so much for all the info

[@fcpeuro]

Thank you for watching!

[@xyphur]

Did replacing the master solve the issue? I ask because I had (and am still having) the same issue on a Volvo XC90 V8, but replacing the master with a new one hasn't solved it. I've been through probably a dozen liters of fluid, bleeding manually (with a lot of pedal pressure so as to quickly flush out any air that may be buoyantly trapped at a high point in the hard lines), power bleeding, gravity bleeding, and reverse bleeding, replaced the ABS module (based on others' experience that the integrated accumulator - specifically its control valve - can become faulty/dirty and thus not keep the accumulator isolated from the system when ABS is not engaged, causing a spongy, sinking pedal), and as mentioned above, replaced the master cylinder with a brand new unit from FCP (bench bled prior to installation). No external leaks anywhere. Running HEL stainless lines and S60R Brembos that are freshly rebuilt with all new pistons and seals. The pedal should feel rock solid and have minimal travel, but it doesn't when the engine is running (or when the key is in pos. II, which enables the aux vacuum pump). Only when the engine is off and key off does the pedal feel firm. The only parts left to replace are the hard lines, and the booster (which seems to be functioning fine; correct power assist, and no vacuum leaks). Any suggestions?

[@gregreed2713]

I'm not familiar with that exact ABS unit, but often times you need a specific procedure to bleed an ABS. In the factory, there is a special program to cycle the solenoid valves on and off. But sometimes, ABS units are 'prefilled', to avoid that need. May be worth checking in your case.

[@xyphur]

@@gregreed2713 I've used VIDA (Volvo's factory diagnostics and service software) to cycle the ABS unit's pump and solenoids while bleeding, no change unfortunately. Perhaps it's just a stubborn pocket of air trapped in a high point in the hard lines. There's one section that I'm suspecting that crosses from one side of the tunnel to the other over the top of the driveshaft/exhaust just aft of the front subframe. I'm going to give it another go with my Motive pressure bleeder and the key in pos. II so the aux vacuum pump can provide assist to the booster, that way I can get a ton of pressure behind it and hopefully blast that pocket down and through to the caliper. I have decent stopping power (probably more the fact that they're upgraded factory Brembos) but the pedal just isn't there - it feels like I stepping on stack of kitchen sponges where braking effect only starts occurring in the second half of the pedal's travel. A recent bit of work on someone else's car (unrelated to brakes) that required me to do a post-work test drive made me jealous of the pedal feel... it was a Pontiac Vibe! :/ Appreciate the suggestion all the same!

[@Loudfire__]

@xyphur did you ever figure it out I have the same issue out of no where after some spirited driving my pedal when soft as well. Exactly like in this video. Replaced the master and fixed it for a second but my problem was intermittent anyways but now o started my car up and the soft pedal is back.

[@alinnimeni3789]

Are your brake pads worn evenly? If not then it may be from something else

[@xyphur]

@@alinnimeni3789 yes, they're evenly worn. This issue was present from the time the pads were installed new into freshly rebuilt calipers. It was also present before the caliper rebuild, so it isn't related to the rebuild either. Part of me is convinced that it's partially to blame on the pads themselves just having poor initial bite and requiring way more effort to achieve a given amount of friction... They are Akebono 'performance' ceramic pads. I'm not impressed with them aside from the fact that they last a long time and don't cause any dust. Their initial bite is non-existent, and max pedal pressure often doesn't engage ABS, which tells me I'm not getting sufficient friction to maximize the grip available at the tires for braking. They will be going back to FCP for a refund when I've decided what pad to replace them with. It is apparent I'm going to have to give up on clean wheels and go back to a semi-metallic pad material. Ceramics are apparently (just barely) sufficient for a daily grocery getter. Anything else and they're useless. I have yet to try another reverse bleed using a fluid syringe. Hoping that doing so will shove whatever air that's in the lines - if any - back uphill to the master, which should be easier to accomplish than trying to force it down against its natural tendency to float. Also going to try removing calipers one by one during bleed and holding sideways in case there's an air bubble getting caught between the caliper halves in the crossover (where the o-ring is). Appreciate you trying to help!

[@Loudfire__]

I have exact same issue. I replaced the master and my issue was intermittent so it was fine but the car sat for a couple weeks and when I started the car the same thing happened with a brand new master… any ideas. Did this car get fixed with the master?

[@alinnimeni3789]

Check the calliper pins,if they re stuck, then it will brake only on one pad,not both. Check to see if the brake bads are worn evenly,if they re not its the calliper pins. If this is not the case, then check the ABS unit

[@Ronniezim]

Hi - my brakes felt great - and I took car in to swap brake fluid and now pedal is quite soft. Aside from brake fluid level being low, what should I check?

[@timhossfeld7260]

My E38 has a very similar problem to the M5 in this video. No external leaks, no codes, pedal slowly sinks to floor. VERY slowly. Brakes just don't feel like they used to. So I bought a master cylinder from FCP Euro and installed and bleed system using ISTA. Problem remains....any ideas? Could my new master cylinder be faulty? I'm out of ideas, and totally miss the awesome brakes my car used to have !

[@SloopJohnBnine]

You DO need to bench bleed the master cylinder before you install it. Google it. And here's the wub....a slowly sinking to the floor brake pedal is always the master cylinder..it's the only place where fluid leaks around the seals don't actually show up on the floor. If a seal leaks on a slave cylinder, you'll get fluid on the floor.

[@gregreed2713]

It's subtle, but ..... if you have air in the system, the pedal force will still have some increasing force with travel relationship. But if you have a leak internal to the MC, the pedal will slowly sink down under a constant force. If you make one quick, spike apply, and the travel is short - that is also a sign of an internal leak.

[@imOmar909]

Hey where you able to solve your issue? I just replaced a MC on a 2000 m5 after a soft pedal and it was okay for about 30 mins of driving on the way back home it felt as if fluid leaked and I had the soft pedal again. Checked for leaks and saw nothing. I will bleed system again tomorrow but I’m not sure what’s the issue is. I’m thinking possibly abs problem?

[@timhossfeld7260]

I have not. The E38 is not my daily driver so I am not driving it much because of the silly brake issue. Really want my brakes to be like they used to be.....am actually considering taking it to a local Indy....which is weird for me since I've done EVERYTHING on this car since getting it in 2008. My current theory is something is wrong with the pre-charge pump or the ABS block...somewhere fluid is getting past a check-valve or something. I'm not sure if the ABS block or pre-charge pump has rubber seals that could have failed or not. @@imOmar909

[@SteWood]

My e90 335d has the exact same brake pedal issue i have changed discs and pads done a full brake fluid change and still the same. so now I’m wondering if the master cylinder is toast.

[@Kro_man_tx]

Thats my issue, the brake booster seems to be working because it is easy to push the pedal. But the pedal travel is crazy. You have to push the pedal several inches before you get any engagement.

[@abdullahal-maliki5318]

What about an internal leak in the ABS hydraulics, can't that cause the same symptoms as a worn out master cylinder?

[@markwhitehead8657]

Yes. That can also cause same symptoms. This happened to my Ford Edge. ABS hydraulic unit was bad. Master cylinder was fine.

[@Ransomed77]

I was driving downhill, hit a small bump while pressing the brakes. My pedal went to the floor, but I could still stop. I drove that way for several miles. Then, while driving up a steep hill, the pedal returned to its normal feel. I checked for leaks, and the fluid level is normal in the master cylinder. Any thoughts?

[@deviouslaw]

I know for my 8p A3, I had a soft pedal that largely resolved when I replaced the hose to the vacuum assisted brake booster. I'm still looking for the cause of my lean condition but the brakes feel a lot better.

[@fcpeuro]

That is a common one on the 8P generation and others - Good luck on finding the cause of your lean condition.

[@JustinDesilva]

I’m trouble shooting the same problem on my A3 8P. I purchased the car last year and since purchasing the brake pedal never felt 100%, more like 80%. 1. The brake booster itself had a leak so I replaced it. You could hear the leak from inside the cabin. 2. I bled the brakes with two people, one person pumping and the other loosening and tightening. 3. Recently I replaced the rear calipers because the pistons weren’t retracting freely. 4. I performed a brake bleed using the Motive bleeder and VCDS. Followed by the traditional bleed method but only by myself, not two people. During the first start up and 30min drive the brakes felt solid, 100%. When driving back home the pedal felt soft again. I will check for a vacuum leak at the hose but I’m thinking the soft pedal is due to air still in the system or damaged seals in the master cylinder.

[@StrangeTimesIndeed]

When I start my car & step on brake pedal it sinks very low. When I put car in "Drive" pedal is firm & brakes operate normally. Pedal only goes low when I'm in "Park" engine running. What could this be? Mc?

[@451oneandonly]

what if you have already replaced the brake master cylinder 3 times and the brake booster 1 time, and replaced 1 wheel caliper that was seized up yet your still having spongy brakes and have a leak coming from one of the studs coming out from the brake booster that holds the master cylinder on.

[@thefishguy8229]

Your brake booster push rod probably needs adjusted, the tool is $15 on Amazon and it's easy to do, did you bleed the master cylinder before putting it on, and did bleed the system after replacing those

[@Conquerred512]

Insanely useful. Have to diagnose the brakes on my track e46 330. Same 'mostly' ok. But it just goes forever...we bled it many many times. Same feel of "i have stopping power, but the pedal never wants to compress and firm up". That said, I think my booster is also going? While sitting, you hear a slight "hissing" sound. With a light to moderate brake press, the hiss sounds like constant hissing air (also a 'push' sound like ur pumping an air pump), but the hiss with keep going. Press further and the hiss/air whooshing stops. I assume that its the booster leaking in addition so plan to replace both the master & booster. Sound about right?

[@gregreed2713]

That for sure is a leaking vacuum booster, definitely should replace!

[@Conquerred512]

Yea. I figured and I think the master is gone too. Doesn't firm up til you floor. Just gonna replace both. In for a penny. In for a pound!

[@gregreed2713]

@@Conquerred512 Good luck. If I recall, 3 series was always Conti-Teves business, not TRW, so yeah, more likely it all failed 😂

[@Conquerred512]

@@gregreed2713 ouch. Ate so yea. Conti... It's fine 160k miles. And track abused. Worth it.

[@melusincube7863]

Did it solve your problem?

[@EmmaRides]

Yoooo Garret! Looking super fine! Great weight lost. So handsome.

[@carlodonnell146]

at this point it seems like its by-passing...because its not going all the way down fast...which a leak will do.......testing myself......Ill watch the whole video now to see what you find

[@mikebell1961]

They make a pill for "Soft Brake Pedal"

[@wilsonrawlin8547]

Agree. Master cylinder slowly going to crap.

[@lockhamj]

I got no brakes at all, i rolled thru a damn red light 😢. Constant beeping sound

[@1marcelfilms]

I just swapped brake booster and master cylinder with one from junkyard car. Problem solved

[@Grrrnthumb]

"It feels solid but..." I do not think that word solid means what you think it means