Hydraulic circuit help?

[Ferret]

This could possibly be one for shogun as the 7's are more common with this type of hydraulic circuit, but...

On the E31, I have the hydraulic brake servo system, however I'm having some "fun" with it now.

During normal braking, the brakes work absoloutely fine, the booster works correctly and everything's a-ok. (aka, slow, smooth movements of the brake pedal)

However, if you hit the brakes hard as in to do an emergency stop, the fun begins. One of two things will happen -

If it's not that serious, you'll get a split second period where the brake pedal resists being pushed (like there's no hydraulic assist) and then the pedal will sink normally (but slowly) after that and the brakes come in fully as usual.

If it's -serious-, the I'm-going-in-the-back-of-that-truck type serious, if you stand on the pedal -really- hard you get zero assist during the whole braking cycle. You have to stop the car on sheer brute force to the pedal and you end up with a twisted ankle.

I thought I had a caliper sticking so I've replaced the brakes this weekend, while it's improved things considerably (the pedal's much softer now during normal braking, like my 540 used to be) - it still does it if you try to brake hard and suddenly.

My question is, does anyone know what this is likely to be? I've been warned that it could be the hydraulic accumulator as at 19 years old, it's likely to have died. This would mean the pump can supply pressure for slow movements of the pedal, but for sudden rapid movements, there's just not enough flow from the pump to cope?

My other "suspicion" is the pressure regulator, as these cars were originally fitted with the CHF7.1 non synthetic fluid, and loads have had the wrong fluid put in causing the fluid to congeal and block up stuff.

I may start with the accumulator, as they're only a cheap part and regularly available...

[Ferret]

Hmm, looking at this it may well be the accumulator:
http://home.insightbb.com/~todd.kenyon/bomb.htm

[whiskychaser]

Any chance it could be the ABS cutting in and not releasing? That would shove your foot back and not allow you to reapply normal pressure to the pedal. IIRC its operated by a solenoid so maybe thats sticking or a return spring is weak? I hope you carry a change of underwear

[Ferret]

Any chance it could be the ABS cutting in and not releasing? That would shove your foot back and not allow you to reapply normal pressure to the pedal. IIRC its operated by a solenoid so maybe thats sticking or a return spring is weak? I hope you carry a change of underwear

Hahaha, yeah nearly needed a change the first time it did it!

Pretty sure it's not the ABS as the pedal feel from the outlay is just wrong - there's no kickback on the pedal, it just wont move!

I've just been to Greedy Screwup F****** to order a new accumulator, the next problem is finding the damn thing on the car! TIS here I come...