Wedge Brakes, Spongy Pedal, final solution

[LLantrisant]

when i purchased my wedge 280i last year and after it was delivered, i recognized that the brakes were binding.(both fronts , 1 rear), brake efficiancy on rear next to zero, handbrake next to zero.

car was freshly moted from the former owner, by the way

investigations turned out that due to storage and hardly any use (200miles in 5 years) the calipers were sticky.
brake pedal was rock-hard by the way.

i freed the calipers, installed new pads all around (incl. handbrake pads).

at the end the brake-pedal was extremely spongy and only became hard after 50% of pedal travel..even i did not open the hyraulics.

i bled the brakes (multiple times, by pressure, by pumping, by vacuum)...no result...pedal remained spongy.
which is unlogic...with seized calipers it was rock-hard..this means the hydraulic circuit is leak-free and without air.

and what was the solution:

i installed a new mastercylinder, bled the brakes and the pedal was again hard.
(even the old Master could be refubrished....but 15,-pound for refurbish kit vs 38pound for a new master...the decission was easy),

so what is the conclusion of all this: sometimes things which seem logic are not.

[phillpot]

Things don't always follow logic!

From your description of the condition of the brakes I would have stripped the calipers and replaced seals as a minimum, pistons as necessary?

Edited by phillpot on Monday 8th March 15:59

[NMNeil]

Two things cause this problem.
The master cylinder is cast iron and the brake fluid is hygroscopic; that is, it absorbs water from the air.
If the fluid is not changed on a fairly regular basis the water content of the fluid gets so high that it will corrode the bore of the master cylinder and/or the piston, which will abrade the piston seal and allow fluid to leak past, giving that spongy brake feel.
The same is true of cast iron wheel cylinders and calipers.

[adam quantrill]

Yes and when you freed the callipers this allowed the master cylinder internals to move - out of a spot where it was sealed good, to where it leaked a bit.

So - we can bring some logic to it after all.

[LLantrisant]

yes...the last reply sounds logic...if, i wouldnt have disassembled the old master and found a perfect bore without corrosion ...correct is that it must have leaked internaly, as there was no fluide leaking from the back into the servo, also the selas looked quite "flat"

[adam quantrill]

Maybe it's still good enough for a rebuild then with new seals?