Urq brake assist is not working properly

[Gonza]

Hi
The other day I changed the rear caliper on my UrQuattro (1991, RR 20v, ABS) and got the air out of the system wrong or some air is stil trap in the system.
The brake pedal slack. If I pump it with the engine turned off, it only gets about 45%hard. And when the engine is turned on, the pedal only sinks a little.

At the same time the "brake" light stays on for some secunds after turning the engine on.
I've been reading about the "bomb" an hope it's not broken....

Hope someone in here may have some answers to my troubles or what could be wrong and how to fix the problem

Time is short, as I'm going to a trackday the 28th of may

Cheers Kasper

[URquattroGus]

Search for Phil Paynes site "isham research quattro" and do the brake tests there.

It should take something like 10 pumps to discharge the bomb.

Hi
The other day I changed the rear caliper on my UrQuattro (1991, RR 20v, ABS) and got the air out of the system wrong or some air is stil trap in the system.
The brake pedal slack. If I pump it with the engine turned off, it only gets about 45%hard. And when the engine is turned on, the pedal only sinks a little.

They can be tough to bleed, and you need to do it in the correct order. Usually takes me a couple of goes. Once I left a broom handle pushing down the pedal for a while to try and get the last bit out and that worked.

Check your brake proprtioning valve for leaks too. Had to replace mine, it suddenly went and was letting air into the system and pissing out fluid.



At the same time the "brake" light stays on for some secunds after turning the engine on.
I've been reading about the "bomb" an hope it's not broken....

The brake bomb assist circuit and actual brake fluid circuit are of course, seperate. The brake light staying on is probably due to either of the two pressure switches on the valve block where to bomb is being defective, or a defective bomb

Hope someone in here may have some answers to my troubles or what could be wrong and how to fix the problem

Time is short, as I'm going to a trackday the 28th of may

Cheers Kasper

Anyway, look at Phil's site and do the bomb test, then maybe look at replacing the pressure switches.

This is a seperate circuit to the master cylinder though, so the air in system is a bleeding , master cylinder or brake prop valve issue

I Pretty much replaced everything in my system, even the brake servo, as this was losing prsssure slightly and was inconsistent in feel.

[daren]

Don't know if the rr has one, have you bled the nipple on the master cylinder?

[URquattroGus]

Everything is here;

http://www.isham-research.co.uk/quattro/urquattro_brakes.html

RR runs different fluid etc but still has a bomb and is almost identical.

[Gonza]

I've used Dot4 fluid and bled the system again, and loosent the handbrake as I should.
We found some more air at the RHS rear wheel. So now everything is a little better But it still dosen't feel perfect...
This evening I found this on a web shop:

"These units also better know as the "bomb" are know to go bad about every 7-9 years of service. IF your brake light ( ! ) keeps going on when you first start the car or stays on for a longer period of time it about time to change the accumulator."

http://sites.internet.lu/folders/customautocraft/typeB3-B4_80_90_S2_RS2.htm#BRAKES

So I should probably change it at the next opportunity ... But I need to get hold of it first

Thanks for the answers bw

[URquattroGus]

No problem. Just make sure you do the pump test to see if it really is the bomb, i.e when the engine is off, how many pumps until it is discharged and pedal goes hard.

It's only about £50 for one (or at least it was a couple of years ago), the same bomb was fitted to some BMW's apparently.