Brake Pedal Pushing Back

#1
Hi Guys,
I'm starting to do some work on the brakes of my 1971 3500. One unusual thing that happens with the car is that when I'm stopped at traffic lights the brake pedal pushes back up against my foot. The brakes remain on and the car doesn't move.....but it's a bit disconcerting. I've never felt this on a car before. Any thoughts or fixes as I start tackling the pads?
Best
John
 

Penguin

Active Member
#2
if it does it stationary it has to be servo related, it could be the servo itself at fault, or possibly the one way vacuum valve the the hose from the engine attaches to or the hose from the engine to the servo leaking somewhere
 

roverp480

Active Member
#3
As mentioned, it sounds like a leak in the servo or part of the pipework or none return valve. If you blip the throttle, does the pedal go down again ?. Hold brake pedal with left foot and blip throttle with right to try this , it increases the manifold depression. .
 
#5
Good day Gents.


I wonder if someone can help or confirm. Brakes !
It seems that when reading the "threads" etc. my problem does indeed point to the servo.
To be clear, no work has been done recently on the brakes and this problem has only very recently occurred.

When on very low tick-over / revs the pedal is very hard and this has indeed minimal braking effect, this discovered when coaring up to a halt at lights and brakes didn't work resulting in very nearly soiled underwear.
The braking and proper feeling pedal is OK when at revs.

I have checked for pipe leakage and have had the small plastic valve mechanism adrift and all looks in order.
The spring does look a bit rusty however but don't know if a weakened spring would give this effect.
Really I want someone to confirm if its the servo or not and If I need to dig deeper.
Looking at the other "threads" then I see problems with master/slave cylinders but as I mentioned, nothing has been done here to interfere.
Brake fluid was changed in the early spring along with one new/reconditioned rear caliper and all working perfectly for the last 6 months.

Last, where and best (price) to get a new unit - 1975 P6 3500S.

Thanks in advance, Stuart.
 

sdibbers

Well-Known Member
#8
My guess would be the air pressure side of the servo is at fault. Vacuum needs to be present to activate the servo and ‘press the pedal’. Under normal servo conditions with the pedal not depressed you would have an equal vacuum on each side of the diaphragm. When you depress the pedal an air valve should open and allow air pressure into the bulkhead side of the servo - That pressure change between the two sides add the extra push of the servo.

My guess would be something is leaking on the air valve circuit side of things. So check the rubber diaphragm under the white plastic ‘flying saucer’, the air pipe going from that to the vacuum chamber with the big diaphragm etc.
 
#9
Many thanks Gents.
I had a look at the small diaphragm and it is intact and still fairly pliable.- no cracks etc.
Also all the hoses are in good condition (looking externally at least) and they have been as fitted new to the car when it was rebuilt 20 odd years ago. This the same age as the servo. Albeit the car has only covered about 25K miles since the rebuild.
I know that the rubber hoses will deteriorate but as I mentioned, they all look good.
I think I will try roverp480 's advise first of all and see what happens.
Looking at the kits themselves, they seem hellish expensive for what you get ! Or is it just me being a miserable old sh#t.
Thanks again lads,
Rgds
Stuart
 

SydneyRoverP6B

Well-Known Member
Staff member
#10
Hi Guys,
I'm starting to do some work on the brakes of my 1971 3500. One unusual thing that happens with the car is that when I'm stopped at traffic lights the brake pedal pushes back up against my foot. The brakes remain on and the car doesn't move.....but it's a bit disconcerting. I've never felt this on a car before. Any thoughts or fixes as I start tackling the pads?
Best
John
Hi John,

From having experienced exactly what you describe on more than one occasion from more than one booster, I can say without hesitation that the problem is the booster itself. Putting a new kit through it or buying an exchange unit that has been rebuilt correctly will see the problem disappear.
Whilst I appreciate that booster kits are not cheap, I advise most forcefully to rebuild the unit so as to avoid an accident.

Ron
 
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