Pulsating brake pedal

BobTaylor

Member
hi, I have just bought a 1972 P6 which hasn't had much use in the last few years. The brake pedal goes up and down when braking, it seems worse when just feathering the brakes at low speed.
My first thought was warped discs, but is it possible it could be sticky calipers?
Bob
 
sticky calipers will either keep pads off or on disc. warped discs can give vibration. as to up and down movement thats normally cars with ABS systems an d these don not have them . wondering if servo valve is faulty here giving a pulsing action? combined with vacuum of engine. we can pop wheels off and check run out on discs. would need to be quite a warp to have such an effect and would have thought steering also affected .
 
Hi, Check the condition of the disc, they may be rusty through inactivity and the pulsing comes from the pads running over the rusty and non rusty parts from where the pads were. Cheap first check.

Colin
 
When you say the pedal goes up and down Bob, by what amount are we talking? Is this oscillation accompanied by a reduction in braking performance? If the pedal pushes back at you, and it will be quite clear that it is, then that is indicative of problems with the booster. Otherwise warped from discs are a distinct possibility. Given it has not had much use for the past few years, it would be advisable as Gerald indicated, to change all of the fluid within the braking system, as a matter of course.

Ron.
 
When you say the pedal goes up and down Bob, by what amount are we talking? Is this oscillation accompanied by a reduction in braking performance? If the pedal pushes back at you, and it will be quite clear that it is, then that is indicative of problems with the booster. Otherwise warped from discs are a distinct possibility. Given it has not had much use for the past few years, it would be advisable as Gerald indicated, to change all of the fluid within the braking system, as a matter of course.

Ron.
HI, the pedal does not push back it just feels like it is going up and down by about 1 or 2 cms. When braking from 30 mph plus it is less noticeable, but feels worse when you just feather the brakes at low speed.
I will take the front wheels off first to see if there is anything obvious like rusty discs or disc run out. I will also take the advice given and change the fluid to start afresh. I do have another servo that came with the car that is supposed to have been reconditioned. Is there any checks I can do with it before I consider swapping them over?
Bob
 
While you are looking at the discs, P6 rears have a nasty habit of cracking in a circular fashion close to the hub, not saying it is your problem but it is a common thing and can cause your symptoms.

Graeme
 
HI, the pedal does not push back it just feels like it is going up and down by about 1 or 2 cms. When braking from 30 mph plus it is less noticeable, but feels worse when you just feather the brakes at low speed.
I will take the front wheels off first to see if there is anything obvious like rusty discs or disc run out. I will also take the advice given and change the fluid to start afresh. I do have another servo that came with the car that is supposed to have been reconditioned. Is there any checks I can do with it before I consider swapping them over?
Bob

It is not possible to check the operational condition of a booster by yourself. I have seen a test bench complete with instruments for conducting operational tests on boosters, but that was in a business that specialised in overhauling boosters. It may well be distortion of your front discs that is the problem, but that is not a common occurrence with the Rover.

Ron.
 
Bleed the brakes and mic the discs up for thickness. If you have the meat to spare, have them skimmed to get rid of the surface defects from standing.
I had a Honda Ballade that was doing the same and new discs fixed it. Only went new because they were cheaper than skimming..
 
I took my car to my local friendly garage and put the car on the rolling brake tester. The front brakes are fine and balanced. The rear are the ones causing the problem, this would suggest warped discs rather than a problem with the servo. So looks like disc replacement or skimming. Never changed inboard discs before, but I have read the article somewhere on this forum on how to do it. I will let you know how it goes.
Bob
 
As said previously by ghce, the rear discs can crack around between the mating face on the hub and the pad surface area. If you get rust in the crack it distorts the disc out of line. With the discs removed, drop them flat-on, outside face down, onto a concrete floor from about 2' high and if they're cracked they'll break or the crack will open up.
 
After further investigation I am still none the wiser. As said before the front brakes perform fine and so I assume that it is not a servo issue.
I have inspected the rear discs which are clean and not scored, I have also checked against an improvised dial gauge and they do not seem warped or distorted.
I tried applying the handbrake whilst driving and seem to get the same uneven braking as when using the foot pedal. I would correct what I said before in that the pedal does not really go up and down, it is just a vibration or juddering when braking.
Any more ideas?
 
If you get the same vibration on the footbrake and when pulling up the handbrake then it's definitely a problem on the rear, and my guess is it's the discs. I can't think of anything on the calipers themselves that would cause it. Even if the pivot pins were seized it wouldn't give you that problem.

Easy way to prove the point is fit a set of known good used discs.
 
If you get the same vibration on the footbrake and when pulling up the handbrake then it's definitely a problem on the rear, and my guess is it's the discs. I can't think of anything on the calipers themselves that would cause it. Even if the pivot pins were seized it wouldn't give you that problem.

Easy way to prove the point is fit a set of known good used discs.
I agree with the logic that if the handbrake produces the same symptoms it must be the discs.
Would you advise buying new or skimming existing?
 
If you're unsure I'd suggest to initially prove the point I'd fit a used pair. If that cures it I'd fit new. If you're sure then bite the bullet, fit new, and hope....

If the car has been standing unused for any length of time they rust on all but the portion covered by the pads. Add to the the possibility of cracks which are common.
 
If you still have doubts, post up some pics of the calipers/discs/whatever you can get a camera aimed at, and I'll have a look and see if there's anything obvious.
 
Hi, Does it do it when reversing? If so then it may be the diff front mounting. Braking and reversing cause the nose of the diff to go down and if the mounting rubber is soft the propshaft bolts clip the crossmember, check also the spacer on the through bolt is present as well.

Colin
 
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