Intermittent brake pedal travel

[I don't]

Occasionally, when I gently depress the brake pedal I get nothing! Normal braking is ok but I've got no idea anymore where the bite point is. Had the garage renew and bleed the brake fluid, pads have plenty of meat etc. I'm thinking it's electrical only because of its random failure but I don't get anything on odb11. Anyone guru who could point at something to try would be grateful.

 

[Pete C]

Sounds to me like a worn/faulty seal in the master cylinder.

Pete

 

[I don't]

My thoughts too at first but the infrequency of it???

 

[alex_p]

I’ve got another car that does this after standing idle for a week or two - if you push gently on the pedal it goes all the way to the floor as the seal doesn’t “fully inflate” in the bore.

What I’ve found is an initial “brake test”, by doing an emergency stop, as I come out of the drive sets the seal properly in the bore and I then don’t have an issue until the car is left parked for a couple of weeks. Doesn’t seem to get worse and I’ve changed the master cylinder on that car before and it is a terrible experience (twin servos with almost quadruple redundancy), so I’ll live with it.

 

[DXX]

I’ve got another car that does this after standing idle for a week or two - if you push gently on the pedal it goes all the way to the floor as the seal doesn’t “fully inflate” in the bore.

What I’ve found is an initial “brake test”, by doing an emergency stop, as I come out of the drive sets the seal properly in the bore and I then don’t have an issue until the car is left parked for a couple of weeks. Doesn’t seem to get worse and I’ve changed the master cylinder on that car before and it is a terrible experience (twin servos with almost quadruple redundancy), so I’ll live with it.

Seriously your are ‘going to live with’ unreliable brakes?? It’s needs repairing, if you’re not to bothered about your own welfare at least think of others.
Degraded seals don’t get better with age.

 

[alex_p]

Seriously your are ‘going to live with’ unreliable brakes?? It’s needs repairing, if you’re not to bothered about your own welfare at least think of others.
Degraded seals don’t get better with age.

They are not unreliable or degraded, it is a new master cylinder - a well known residual pressure imbalance in the system pushes fluid against the seal and bypasses it when the car isn’t driven for weeks at a time. Applying the breaks once puts the seal back against the walls and it performs 100% every time. The car sees significant track use, so the brakes see far more strain than a road car.

I mention it purely because the OP reported symptoms that sound a lot like fluid pushing past the seal in a master cylinder - something worth checking out and understanding, and starting with the master cylinder would be a good shout in my opinion.

 

[I don't]

Sorry been away (not in the van!!). I've not driven her until I get this fixed and I'm happy to go to a garage to get it done properly (yes I care about others both in and out the van). I'm just asking as I'm not too knowledgeable under this bonnet and was told by the mechanic it could be the vacuum pump or the seals etc so without replacing everything from the pedal to the pads I was hoping for a "this bit is known to fail so try this first"

 

[alex_p]

Could be a servo failure - can you “pump up” the brakes? i.e. if you operate the pedal a couple of times does it travel less and become “hard”? With some cars you can start the engine with your foot on the brake and feel if the servo starts to “suck”…

Are you losing fluid? I don’t know enough about the van brakes specifically, however, do have a fair bit of experience with braking systems generally:

1) If the fluid isn’t coming out on to the floor and the reservoir isn’t emptying, it’s probably the master cylinder seals bypassing between the front and back circuits

2) If the fluid isn’t coming out on to the floor and the reservoir is emptying, try the servo (a lot of fluid can hide in there!)

3) If the fluid is coming out on the floor then you’ve found your problem! Now is a good time to check if the van hasn’t moved for a while - I’ve seen brake wheel cylinders leak and pool fluid in a wheel, so have a good feel around slave cylinders as well as looking.

If you’re not confident with brakes and hydraulic systems, probably best not to mess with them, as bleeding and chasing out airlocks can be hard/heartbreaking and/or they can come back at an inconvenient moment…

 

[I don't]

Yes, pumping the pedal hardens up. No drop in the tank level or puddles on the floor and backed her onto ramps and couldn't see any bulges while the misses applied the brake. Anyway she booked in for the master cylinder next week and fingers crossed that'll do it. The mechanic pointed to a small filter/pump clipped to the trim bottom center of the windscreen area under the bonnet. Is this my vacuum pump??

 

[alex_p]

The bit with the lid on is the reservoir, and this sits on top of (and feeds into) the master cylinder - you’ll notice there are two feeds in from the reservoir - one for each braking circuit, usually one front wheel and the back wheel on the opposite side, to prevent total brake failure if you had a ruptured line and pumped all the fluid out. At the front of the master cylinder is a big black round thing - this is the servo, it’s “just” a rubber diaphragm which has a vacuum “sucking” on one side from the engine that then multiplies the effort of you pushing the pedal - have a look for a YouTube video, as it is really simple to watch but hard to explain as clearly!

Changing the master cylinder isn’t too bad of a job if it’s quite high up in the system, as it limits the amount of air that can get in and need to be bled out, likewise the servo can be removed, opened and inspected to see if the seals are good - I’ve never had much luck rebuilding them with new diaphragms so tend to replace them, if they’re leaking/have fluid on the wrong side.

 

[I don't]

New master cylinder chucked in and problem solved! Bit concerning as it's only done 67k