Ur quattro Brakes Binding & Hard Pedal - Page 2

Tun · 14-03-17, 03:51 PM

I believe this is a common problem getting with the UR's. Replace my master cylinder 2 years ago, if I got caught in traffic and found myself constantly on and off the brakes they would pump up and bind. Leaving the car alone for 20 or so minutes would allow me to carry on. I suspected the bomb at the time because the brake pedal would go hard after only a few pumps (which it still does!) but the culprit was the master cylinder.

Hanuman · 14-03-17, 05:16 PM

I couldn't help but chuck in a little pic, albeit from a different vehicle, but demonstrates how the system is common across many manufacturers.

The Hydraulic reservoir holds hydraulic fluid, through the valve block.
The Pump pressurises the hydraulic system, Orange circuit.
Residual fluid is returned to the reservoir, Orange circuit.
The Steering is supplied by one outlet, Red circuit.
The Brakes are boosted by another outlet, Blue circuit.
The Hydraulic system is backed up by the pressure reservoir/sphere(bomb). It's described as the brake bomb, which I've often thought was slightly eroneous; but I guess not, now that I've seen the document the Jim has posted after this.

The master cylinder operates the brakes, Yellow, hydraulically boosted.

jimh · 15-03-17, 12:01 AM

The brake assist / bomb part of the hydraulic circuit is very similar to the aircraft system described above.
The valve block on the bottom of the bomb (shown on top of the bomb in the illustration), splits the pump flow between steering and brakes.
The steering has the majority of the flow, the brakes require much less flow.
The bomb is a pressure reserve for the brakes hydraulic circuit only (the flow required by the steering would drain the bomb very quickly).
The brake assist is always fed direct from the bomb. The valve block supplies flow from the pump to recharge the bomb.
There's a good explanation of the system here, starting at page 42
http://www.quattro.ca/Urq/urq-IST-Introduction.pdf

For the OP - replace the master cylinder. That should fix the binding brakes issue (and is a lot cheaper than the servo).
Then do a bomb test with the new master cylinder. If the results of the bomb test are still poor, it's time to replace that too.

Also bear in mind the age of the pump. I had a spare second hand pump laying on the shelf for years, then decided to try it out on a whim. It gives noticeably stronger brake assist than the original, so it stayed on the car.

Benjamino · 15-03-17, 09:01 AM

Thanks Jim.

I can add another couple of symptoms into the pot which are as follows;

On first start up after a few days rest, the red warning light is on until I get to the end of my road (100m or so) and then goes off. I think it has always done this in the 8 years I have owned it and I just assumed this was normal.

Also, the brakes can be quite unresponsive until this point too and a hard press of the pedal would be needed to stop the car. I always assumed that this was due to cold pads!

Does this give any clearer clues as to pinpoint which part is on its way out?

Cheers

mikes2 · 15-03-17, 12:13 PM

When I had that on my MB, it was a servo problem. If you pump the pedal, the light will go out. It's losing pressure if it sits. On mine the pump and the bomb were new

I never had a brake binding problem though.

Fitting the replacement transformed the brakes

jimh · 15-03-17, 06:48 PM

Sounds like you have a brake binding problem, and a generally rubbish brakes problem too...

The brakes coming on by themselves and binding problem is at least 9/10 times the master cylinder. Once or twice over the years I have heard of a bad servo causing this, but servos are big bucks!

I'm assuming MB, so no difference in warnings for fluid low/handbrake on/hydraulic pressure low - just one red warning light for any of those problems.

The general poor brakes / taking a long time to charge sounds like bad bomb, or low pump output.
Does the MB have the twin output pump like the RR? (separate pump outputs for steering & brakes). If so, we cannot rule out bad pump just because the steering feels ok.
Could also be the servo at fault.

There is no way to diagnose which part of the system has failed, or is under performing (probably all of it) without (very) specialist high pressure test equipment, apart from best guesses from previous knowledge of similar / same sounding faults, bearing in mind the cost of some of these parts.

I think I would:
Replace the master cylinder (see if the binding issue is resolved).
If the brakes no longer bind, replace bomb (see if the slow charge / poor assist is resolved).
If slow charge is not resolved, replace pump (see if you can get a second hand one or borrow one for testing).
If slow charge still not resolved, replace servo

If the brakes still bind after replacing the master cylinder and/or the slow charge is never resolved, it's time to seriously consider replacing the servo, I think it's priced in the thousands.

There's an outside chance that you could replace the master cylinder and the bomb and the pump and still end up replacing the servo, but if I had to spend that much on a servo, I think I'd like to run it on new pump/bomb/master cylinder/fluids anyway...

Benjamino · 16-03-17, 10:06 AM

Thanks Jim - that's very much appreciated. I've ordered the Master Cylinder from Pete Reeve so I'll take it from there and hope for the best!

14-03-17, 03:51 PM	#11
Tun Grown up member Classic Audi Club Member Join Date: May 2011 Location: Buxton Posts: 745	I believe this is a common problem getting with the UR's. Replace my master cylinder 2 years ago, if I got caught in traffic and found myself constantly on and off the brakes they would pump up and bind. Leaving the car alone for 20 or so minutes would allow me to carry on. I suspected the bomb at the time because the brake pedal would go hard after only a few pumps (which it still does!) but the culprit was the master cylinder. __________________ 1988 MB quattro 2012 S4 Avant

14-03-17, 05:16 PM	#12
Hanuman Trickster Classic Audi Club Member Join Date: May 2009 Location: Heckling from the cheap seats, Phnom Penh, KoW Posts: 7,006	I couldn't help but chuck in a little pic, albeit from a different vehicle, but demonstrates how the system is common across many manufacturers. The Hydraulic reservoir holds hydraulic fluid, through the valve block. The Pump pressurises the hydraulic system, Orange circuit. Residual fluid is returned to the reservoir, Orange circuit. The Steering is supplied by one outlet, Red circuit. The Brakes are boosted by another outlet, Blue circuit. The Hydraulic system is backed up by the pressure reservoir/sphere(bomb). It's described as the brake bomb, which I've often thought was slightly eroneous; but I guess not, now that I've seen the document the Jim has posted after this. The master cylinder operates the brakes, Yellow, hydraulically boosted. __________________ I wish they would keep the damned Chinese away now that I can go home, so that I can enjoy Fish amok and a draught Angkor Last edited by Hanuman; 18-03-17 at 08:47 AM. Reason: New info

15-03-17, 12:13 PM	#15
mikes2 4 ring whore! Classic Audi Club Member Join Date: Nov 2007 Location: Polmont Posts: 2,617	When I had that on my MB, it was a servo problem. If you pump the pedal, the light will go out. It's losing pressure if it sits. On mine the pump and the bomb were new I never had a brake binding problem though. Fitting the replacement transformed the brakes __________________ 1991 90 quattro 20v sport 1991 90 quattro

15-03-17, 12:01 AM	#13
jimh Grown up member Classic Audi Club Member Join Date: Dec 2008 Posts: 122	The brake assist / bomb part of the hydraulic circuit is very similar to the aircraft system described above. The valve block on the bottom of the bomb (shown on top of the bomb in the illustration), splits the pump flow between steering and brakes. The steering has the majority of the flow, the brakes require much less flow. The bomb is a pressure reserve for the brakes hydraulic circuit only (the flow required by the steering would drain the bomb very quickly). The brake assist is always fed direct from the bomb. The valve block supplies flow from the pump to recharge the bomb. There's a good explanation of the system here, starting at page 42 http://www.quattro.ca/Urq/urq-IST-Introduction.pdf For the OP - replace the master cylinder. That should fix the binding brakes issue (and is a lot cheaper than the servo). Then do a bomb test with the new master cylinder. If the results of the bomb test are still poor, it's time to replace that too. Also bear in mind the age of the pump. I had a spare second hand pump laying on the shelf for years, then decided to try it out on a whim. It gives noticeably stronger brake assist than the original, so it stayed on the car.

15-03-17, 09:01 AM	#14
Benjamino Grown up member Join Date: Jun 2009 Location: Dronfield, Derbyshire Posts: 116	Thanks Jim. I can add another couple of symptoms into the pot which are as follows; On first start up after a few days rest, the red warning light is on until I get to the end of my road (100m or so) and then goes off. I think it has always done this in the 8 years I have owned it and I just assumed this was normal. Also, the brakes can be quite unresponsive until this point too and a hard press of the pedal would be needed to stop the car. I always assumed that this was due to cold pads! Does this give any clearer clues as to pinpoint which part is on its way out? Cheers

15-03-17, 06:48 PM	#16
jimh Grown up member Classic Audi Club Member Join Date: Dec 2008 Posts: 122	Sounds like you have a brake binding problem, and a generally rubbish brakes problem too... The brakes coming on by themselves and binding problem is at least 9/10 times the master cylinder. Once or twice over the years I have heard of a bad servo causing this, but servos are big bucks! I'm assuming MB, so no difference in warnings for fluid low/handbrake on/hydraulic pressure low - just one red warning light for any of those problems. The general poor brakes / taking a long time to charge sounds like bad bomb, or low pump output. Does the MB have the twin output pump like the RR? (separate pump outputs for steering & brakes). If so, we cannot rule out bad pump just because the steering feels ok. Could also be the servo at fault. There is no way to diagnose which part of the system has failed, or is under performing (probably all of it) without (very) specialist high pressure test equipment, apart from best guesses from previous knowledge of similar / same sounding faults, bearing in mind the cost of some of these parts. I think I would: Replace the master cylinder (see if the binding issue is resolved). If the brakes no longer bind, replace bomb (see if the slow charge / poor assist is resolved). If slow charge is not resolved, replace pump (see if you can get a second hand one or borrow one for testing). If slow charge still not resolved, replace servo If the brakes still bind after replacing the master cylinder and/or the slow charge is never resolved, it's time to seriously consider replacing the servo, I think it's priced in the thousands. There's an outside chance that you could replace the master cylinder and the bomb and the pump and still end up replacing the servo, but if I had to spend that much on a servo, I think I'd like to run it on new pump/bomb/master cylinder/fluids anyway...

16-03-17, 10:06 AM	#17
Benjamino Grown up member Join Date: Jun 2009 Location: Dronfield, Derbyshire Posts: 116	Thanks Jim - that's very much appreciated. I've ordered the Master Cylinder from Pete Reeve so I'll take it from there and hope for the best!