No brake pressure on front circuit after master cylinder replacement

[LightlyToasted]

2007 BMW X5 E70 4.8i.


Original problem was a hard brake pedal despite good vacuum at booster so I replaced the booster, which involves removing and reinstalling the master cylinder then bleeding the system (including DSC bleed via ISTA). Brakes had boost again, but pedal would slowly sink to the floor. I suspected that when I bench-bled the master cylinder it may have stressed the old seals, so I admonished myself for not replacing the master cylinder the first time and went ahead and installed a new master cylinder.

After thoroughly pressure bleeding, DSC bleeding via ISTA, and 2-person manual bleeding the pedal will sink almost to the floor with minimal resistance, as if there were a problem with the (just replaced) master cylinder. There are no bubbles in the system and no brake fluid leaks anywhere. When I pressure bleed I get clear fluid at all 4 corners so the circuits aren't blocked. When I 2-person manual bleed, no fluid comes out of the front brakes.

Since the front brake circuit is not blocked (pressure bleed works fine), the problem must be at the master cylinder which is not supplying pressure to the front brake circuit. Any ideas? Could there be air in the front circuit portion of the master cylinder? Bad master cylinder? To be clear, I bench bled the cylinder before installing it (even though the BMW TIS doesn't require it), and additionally I slowly cycled the brakes while pressure and DSC bleeding as per the TIS.

[andrewwynn]

If pedal sinks with no link the only place i can think of is the piston seal for the front circuit.

I've noticed my pedal will very slowly sink under constant pressure so I'm due for MC rebuild.

New ≠ fixed, you have to follow the symptoms even if it means the brand new part is the likely candidate.

It's very strange to have zero pressure.

If you hit the pedal hard but don't hold to you get resistance? (Suggests weak vs. missing o-ring).

[andrewwynn]

If the pipes are long enough you can confirm MC fault by swapping front and rear output and the problem will go to the rear.

(I'm not sure there's not an additional bypass path with the abs pump and block)

[LightlyToasted]

Originally Posted by andrewwynn

If you hit the pedal hard but don't hold to you get resistance? (Suggests weak vs. missing o-ring).

Nope - pedal sinks about 90% to the floor no matter if I press slowly or hard.


Come to think of it, I recall hearing a "pop" noise right after I installed the master cylinder and initially pressed the brakes. I chalked it up to the plunger seating in the piston, but now I wonder if the front circuit seal gave way.


There's no room to swap front and back circuits at the master cylinder - the lines have almost no give, which makes installing the master cylinder a real chore.


Is there any way to bench test the MC? Perhaps plug the lines and see if the piston can be depressed?

[andrewwynn]

That'll work as well as swapping lines.

Plug the front outlet. Press pedal. If it firms up, not MC.

The plungers are usually attached via a mini teeter totter and maybe one side pin broke?

[LightlyToasted]

Removed the MC and bench tested it. Sure enough, the front circuit had failed completely and would not build any pressure at all.


FWIW it was a new Beck Arnley from RockAuto with a 1 year warranty. Let's see how the return process goes...

[80stech]

This isn't really making sense, what was your bench bleeding procedure ? Do you have a kit with hoses to return to the reservoir? By "no pressure" you mean no air either? You were not able to "pump up" the brakes at all when bleeding?

Is the front of the master cylinder also the front brakes ?

[LightlyToasted]

Originally Posted by 80stech

what was your bench bleeding procedure ? Do you have a kit with hoses to return to the reservoir?

Yes, I have that kit. When bench bleeding both circuits were pumping, so I suspect the seal gave way after it was installed on the vehicle. That could have been the "pop" I heard.

Originally Posted by 80stech

By "no pressure" you mean no air either? You were not able to "pump up" the brakes at all when bleeding?

No air either. Not able to pump up the brakes at all - pedal would sink 90% of the way to the floor at which point the rear circuit was pressurized by physical contact of the primary piston (look up how dual circuit master cylinders work when the primary circuit fails).

Originally Posted by 80stech

Is the front of the master cylinder also the front brakes ?

The front circuit is the primary circuit which is closest to the firewall.


New MC is on the way. I will not be bench bleeding this time just in case my bench bleeding procedure which pushes the piston all the way in (perhaps past normal pedal travel?) risks damaging the seals.

[LightlyToasted]

Well guys, I'm losing my effing mind.

Removed the (new, recently-replaced) master cylinder. The front circuit seal had indeed failed completely - it would not build any pressure on the bench. Sent it back for a refund.

Got a replacement master cylinder. It built pressure on both circuits on the bench while bench bleeding, and I was very careful to not over depress the piston in case this could damage a seal. OK, so far so good. Installed it, thoroughly bled the system (pressure, DSC, and manual). Pedal feels firm and holds when vehicle is not running. When the vehicle is running (brake booster on) with firm pedal pressure the pedal will slowly sink 80% to the floor which is the same symptom I had previously where the front circuit seal had failed. Pumping the brakes firms things up, but the pedal will sink to 80% if you continue to press hard. I suspect that this 2nd new master cylinder has a leaking but not outright failed front seal.

Again, no bubbles, no leaks.

Unless someone has any insight, I'm going to order an OE MC from FCP Euro. If that works then I'm going to disassemble the Beck/Arnley MC from RockAuto to see what is wrong with the seal.

[80stech]

Maybe the ABS is leaking fluid by ? Do you have a plug you could use to block off one circuit and see if the pedal still drops ?