Rear brakes do not function at all

[rob07]

All 4 brakes were done 2 years ago and worked fine. But now after a long period of non use the brake pedal goes 80% to the floor before feeling any pressure. Both rear brakes do not work at all and will not even bleed,no fluid comes out when pumping the pedal but does so under gravity.
I think it cant be the brake booster as the rears should still have some function. My understanding of the rear brake pressure regulator is that the bias may be off,but again the rears should still have some function. Which only leaves the master cylinder which does have a front and rear circuits. However I have never experienced complete failure. The seal fuction usually degrades over time namely due to corrosion in the bores.
Any advice welcomed

[GreenV8S]

Seems to me you must have a hydraulic failure in the rear circuit. That could be anywhere in the circuit, including the m/c. The m/c is the only place where you could have a hydraulic failure without also having a leak.

[rob07]

I agree Its likely to be the master cylinder, but as I said how can it fail to the extent that I cant even pump a dribble whilst trying to bleed the brakes.

[E-bmw]

Because the seal is passing to such an extent that no pressure is built up by pumping.

[rob07]

Im not saying its not possible, but how can a seal fail to such an extent when it was working fine not that long ago.

[GreenV8S]

rob07 said:

Im not saying its not possible, but how can a seal fail to such an extent when it was working fine not that long ago.

Easily, if the m/c has built up internal corrosion while it was standing, and that has now ripped the seal.

[stevieturbo]

perhaps give us a clue as to what car ?

and what specifically does brakes being "done" entail ?

[The Road Crew]

stevieturbo said:

perhaps give us a clue as to what car ?

and what specifically does brakes being "done" entail ?

Quite. I do enjoy a guessing game!

As we've no idea about the system in question... How about a seized proportioning valve for the rear axle not letting fluid thru at the correct rate?

[njw1]

GreenV8S said:

rob07 said:

Im not saying its not possible, but how can a seal fail to such an extent when it was working fine not that long ago.

Easily, if the m/c has built up internal corrosion while it was standing, and that has now ripped the seal.

My missus old Fiesta was in daily use yet still suffered a master cylinder failure which resulted in near enough zero brakes, quite scary that was!

[rob07]

I believe I have figured out what is wrong. Its not the master cylinder seal thats failed as that does not correspond with my symptoms. However its the master cylinder piston that is stuck in the depressed mode. The master cylinder is bolted to the booster whiich is bolted to the pedal box which are all buried in the depths underneath the dash. Its a mountain of a job to pull the lot out inc the dash so unless I can figure out a way to free the piston it will have to remain unfixed for a long while yet.

[GreenV8S]

rob07 said:

Its not the master cylinder seal thats failed as that does not correspond with my symptoms.

It does fit the symptoms as you've described them, but a stuck piston might too.

You'd need to disconnect the output from the m/c to remove it in any case. There is just an outside chance that if you apply enough pressure on the output port you can blow the piston back. Just be aware that in the best case if you manage to get it unstuck, the contents of the reservoir will try to spray themselves around your engine bay and there is no reason to think that this has solved whatever problem has caused the piston to stick. You almost certainly have to replace the m/c whichever way you look at it. In all cases I've seen, the m/c bolts to the front of the booster. Are you sure you have to take the dashboard / pedal box route?

[rob07]

I recall reading somewhere that when bleeding the brakes it is prudent to not to push the pedal all tge way to the bottom of its travel as that would push the master cylinder piston further down the bore than it would normally travel,which could lead to a stuck piston or damage to the seals.
I am going to try to push back the fluid via a brake bleed nipple which should work, its just a matter of finding suitable connections and a pumping system.

[Tony1963]

Is the brake pedal stuck at/near the fully depressed position? If not, how can the piston be stuck there?

[Pica-Pica]

I’m confused here. If it’s a diagonal split system, then how does an issue at the master cylinder only affect the rear brakes?

[Maxdecel]

Pica-Pica said:

I’m confused here. If it’s a diagonal split system, then how does an issue at the master cylinder only affect the rear brakes?

Likewise - from OP's first post "Which only leaves the master cylinder which does have a front and rear circuits" So it's modified or quite old as I think ? Iirc? Sierra was the last car to have F/R Split. And I'm far from certain about that !

[rob07]

Yes it is old so not diagonal front and rear.

[Tony1963]

I’ll repeat my earlier question:

Is the brake pedal stuck at/near the fully depressed position? If not, how can the piston be stuck there?

[GreenV8S]

Tony1963 said:

Is the brake pedal stuck at/near the fully depressed position? If not, how can the piston be stuck there?

If it's a tandem master cylinder there are two pistons in series. The further one can get stuck without locking the near one. I don't know whether this is the problem but it's academic at this point since the m/c will need to be replaced in any case.

[Tony1963]

Cheers

[rob07]

Green v8s is correct," it's a tandem master cylinder there are two pistons in series. The further one can get stuck without locking the near one"
Its just too big a job for me to do right now,so I wanted to free the piston for the time being.