Sticky brakes

D

Duane

Active Member
Hey All,

Since getting my P6B on the road I have always suspected the brakes have been temporarily sticking slightly. Symptoms were a sluggishness that would suddenly come right (car surging), and a slight squeak from the front brakes occasionally (as if the pads were just dragging a little). The master cylinder and servo were fully rebuilt with new parts and seals, and when the the engine was not running there was no sign of the symptoms.

I have always suspected the air bleed system on the servo, and have had it apart multiple times (even the new unit purchased from Wins). Particularly the small bleed off piston that is pushed back in by the spring in the valve assembly when you release the brake pedal. I have had that piston out half a dozen times times cleaning the bore, sanding with very fine wet n dry, etc. Anyway, I happened across an old servo from a car being wrecked, and one day thought I'm gonna compare it to mine. I noticed that the piston was much easier to push back in than the one on my car. To prove a point, I put the old piston with it's old seal in mine and it fixed the problem instantly. The car is transformed.

The issue is definitely the new hydraulic seal for that tiny bleed piston (supplied in the servo rebuild kit from Wins) being a fraction oversize for the bore. It is probably only thousandths of an inch but just dragging slightly.

Thought this might help others with the same symptoms as it is almost impossibly to diagnose.
 
yes agree that little piston is a pain if its sticks we had the same on a remote fits all unit on the Vitesse ended up
replacing and yes solved .
I have 2.25 :1 remote we got for the 3500 but it needs the 4.25 to stop the thing so its looking for a good home
we fitted a servo kit from another supplier and the old unit now cleaned and painted and not full of fluid is working well
you could try a stronger spring in the air valve as a trial
having played with many servos the earlier types had a cup seal the later seem more of an 0 ring seal
other things to cause drag is foot pedal pushrod free float and the vacuum piston free play to its hydraulic piston
but that little plunger is the life and sole of the operation
Pete
 
A couple of other things to look at on the slow releasing brakes. Check the brake pedal height @harveyp6 has written a very good guide on that in the brakes section.

I had a similar issue on my car for ages, always assumed it was the over complicated rear calipers. Turned out it sticky pistons in the front calipers. So maybe jack up the car, have someone step on the brakes while you’re outside and try and turn each wheel as your mate releases the brake pedal. That way you can see if it’s caliper related (one or two sticking brakes) or servo, brake pedal adjustment related (all wheels affected).
 
Yep went through all that. Just wanted to point out that tiny 5mm diameter piston and more importantly the modern hydraulic seal that seems to be manufactured slightly oversize. Very hard to diagnose unless you have the original seal to compare the action with.
 
Duane said:
Yep went through all that. Just wanted to point out that tiny 5mm diameter piston and more importantly the modern hydraulic seal that seems to be manufactured slightly oversize. Very hard to diagnose unless you have the original seal to compare the action with.
Click to expand...
When I rebuilt my servoes (nada four bangers have two *yay*) one of the small pistons had rusted into its bore. I was able to remove by drilling and tapping into the top of the piston, screwing a bolt into it and teasing it out.

After that I reamed it with a 3/8” reamer before turning up a new piston. That made a huge difference, the other servo used to stick until I reamed that bore too. So maybe worth a try?
 
There is a modification spring that was/is available that is there to correct the issue with the air control piston from sticking. They were available from brake repair businesses in Sydney during the late 1980s but more so during the 1990s and 2000s. The cover on the air valve chamber is detached and the piston positioned over the rubber diaphragm. It is sized to fit nicely and not skew when the cover is refitted. I have used them for decades and they work perfectly to remove any tendancy for the brakes to stick, even on very light throttle applications. Their dimensions are 40mm in overall length with a diameter of 38 mm. I do not know what the spring constant is but they are not a stiff spring by any stretch.


Ron
 
having just rebuilt the vac unit the plastic air valve and filter is available from many other marques mine cost £19 some sell for nearer £50
just search eg sunbeam alpine , dont get caught
Pete
 
SydneyRoverP6B said:
There is a modification spring that was/is available that is there to correct the issue with the air control piston from sticking. They were available from brake repair businesses in Sydney during the late 1980s but more so during the 1990s and 2000s. The cover on the air valve chamber is detached and the piston positioned over the rubber diaphragm. It is sized to fit nicely and not skew when the cover is refitted. I have used them for decades and they work perfectly to remove any tendancy for the brakes to stick, even on very light throttle applications. Their dimensions are 40mm in overall length with a diameter of 38 mm. I do not know what the spring constant is but they are not a stiff spring by any stretch.


Ron
Click to expand...
Yep mine had that additional spring, but the new seal was still binding in the bore. Old seal is working perfectly.
 
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