Dual brakes sticking on.

Barten

Active Member
My brakes sticks on after the engine has warmed up, and is binding quite severely. Today I bled my brakes as per the manual, rear and one front side at the time, but that did not help. After testing again and brakes were binding, I jacked up the rear and those were free, then jacked up front and they were both binding. When I stopped the engine I tried the brake pedal, I could hear the hiss from the servo air valve 3-4 times when I pressed in the pedal. After that the brakes were free. So it seems to me like the servo air valve is not releasing the pressure before the engine is off. It is obviously something I don't understand here. I have changed the rubber diafragm but not the valve itself. Maybe the clever Rover p6 community can come to my rescue once again? I have read all I can find about dual brakes but did not find anything just like this.
Regards, Barten
 
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I had a similar situation on a P4 Rover , that was the piston in the Servo sticking on, not allowing the fluid back to the reservoir, but as it is a single line system all four brakes were held on. Not being familiar with the V8 set up, Does yours have one or two servos?
 
My 3500s has got 1 servo. The dual circuit work so that front and rear are separated on the car. So somehow the front brakes are stuck on and I suspect the servo air valve has something to do with it.
 
Yes Clive, thats the servo air valve I mentioned. It sits on the end of the master cylinder. Today i have removed the valve and dismantled it. I put in an extra spring in case the original one was too weak, but no change in braking. They stick on when applied with engine on. I turn off engine, then after pumping the pedal 3 -4 times, the wheels free off.
 
The U/S NADA models have the same type and suffer the same fault including mine.
I found on mine the retraction valve piston was sticking.
When I rebuilt mine after fully cleaning and inspection I refitted the seals lubricated with silicone grease and the white plastic bearing also lubricated the same.
Is your car LHD?
Do you have the heat shield over the N/S exhaust manifold?
After the rebuild I thermal wrapped the brake master, seemed to do the trick
Clive.
 
My car is LHD. I do not have a heat Shield installed, but I have ceramic coated manifolds, so the heat should be OK. Is the retraction valve piston the plastic piston Attached to the rubber diaphragm or is it part of the master valve Assembly? I cant find a good drawing of the master cylinder internals. Where is the white plastic bearing sitting? I surely hope we are on the right track her, It feels like we are very Close now.
regards, Barten
 
By the way Clive, I do have the classic car Magazine With HWC 230N on the from page. I remember bying it around 90 - 91 I think. Great inspiration for me! Thanks!!
 
This was a common problem with V8 master cylinder (NADA 3500s in this case) back when I was working on these. Sadly, our solution was a new master cylinder so I can't advise on what bit is sticking, but Clive's description matches my memories.

Yours
Vern
 
Hi Barten.
HWC is now home with me after a long term storage round my mum's.
A couple of pictures of master cylinder air piston.
Note this master is a NOS jaguar one and the only difference is that the rover air piston is 3/8 and jag is 5/16
Regards.
Clive. 20180117_191107.jpg 20180117_191135.jpg 20180117_191242.jpg
 
I have now dismantled my brake master cylinder. I have greased up the part Clive mentioned and also the parts mentioned in the WSM. Refitted everything and bled the brakes. Brakes still binding after applying the pedal with the engine running. Not encouraging!! I may have to bite the bullet and buy a new master cylinder. I think Wins have them. But isnt much fun when I havent found out what the problem is!
 
The only other components which could be suspect is the tandem servo if you have never serviced it and the brass 4 way shuttle valve.
Repair kit for the servo i got from a company called Power Track here in the U/K he is on the net and only sells gen new not NOS seals.
The shuttle valve is a very simple device with a single ring seal.
Got a seal kit from a Triumph stag specialist, name escapes me at the moment.
Regards.
Clive.
 
I tried the car again today. I wanted to check if renewing the air valve had an impact. I removed the filter/spring unit from the air valve to check if manipulating the diapraghm would have any impact. It did not free the brakes completely, but they were not as stuck as before. I also tried to remove the whole air valve and applied the brakes again, then they were really hard stuck on. Put it all back together and tried to run it back and forth a few times, no improvement.
My calipers are overhauled and seem to work well since brakes free off when i press the brake pedal 3-4 times after i shut down the engine. I have also overhauled my 5- way valve recently and it seem to be ok now (no leaks), and the servo was also overhauled. I must mention that I am using automec silicon brake fluid and I have seen others claiming this can be a problem. I see so many advantages with that fluid and i cannot see why it should prevent full function.
 
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I personally would not use the silicon type fluid , have read that if you use silicon fluid you must assemble the cylinders with silicon grease when rebuilding.
 
Given that totally evacuating the servo releases the brakes I can only conclude that the servo when at rest, but with the vacuum applied it is still pressing on the master cylinder pushrod and not allowing the front piston to return to is recuperating position. Something is either incorrectly assembled or adjusted. Is there any adjustment between the Servo and pushrod on the Master cylinder?
 
I have been thinking that something must be assemled wrong in the slave cylinder. On the p6 with dual brakes the slave cylinder is attached to the servo. I think i must check there.
Regards Barten
 
Hi Barten
Does your system look like this?
Although it's a tandem system the master cylinder only has 1 main pressure outlet.
Clive
20180120_192616.jpg 20180120_192224.jpg
 
Yes Clive, it looks like that. I think the nada model is the same. Mine is a 3500s originally sold in Norway. Same spec as they had in Sweden but as far as I know they newer imported the 3500s to Sweden.
Regards Barten
 
The only other problem i found was the push rod for the master cylinder had seized in its pivot on the brake pedal.
Does the pedal assembly look ok , is there clearance on the push rod and master cylinder when brakes released?
I think if it was me i would be taking the master and servo off / stripping and very carefully inspecting and rebuilding again with new kits again using silicon grease to lube the air piston and master cylinder piston plastic bearing.
Both kits were reasonably priced from Power Track and only use dot 4 fluid. See if you can get the exhaust heat shield [ try Federal Rovers site]
I change my brake fluid every year and doing that greatly reduces the chance of any moisture building up and damaging the system.
Regards.
Clive.
 
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