Brakes tend to grab.

[Caracad]

Advice.
The brakes on my S3, tend to grab suddenly when I press the pedal. The brakes seem over sensitive, the servo giving full assistance even on light braking.
However, brake hard and they then need a really good push, as if the servo is not working. The brake force on the pedal is not linear making it hard to modulate braking.
Pedal is nice and firm and the brakes release as they should.

Any ideas anyone?

[lancialulu]

Interesting. Almost seems like a or some collapsed hoses or the pads not moving cleanly in the calipers or the pistons sticking..... Only a thorough reveiw of the system will shed light....

[Caracad]

Thanks Tim,
I have got used to it as is, but will check the system properly when I get some time.
The calipers were refurbished and a new master cylinder fitted before I bought the car and on the whole the brakes are excellent. In fact for a 42 year old car the brakes are incredible.

I am thinking it might be something out of tolerance between servo, master cylinder and pedal.

I also need to check the circuits a plumbed in correctly. 

[chriswgawne]

Sounds like a servo/master cylinder problem to me. Maybe try disconnecting the vacuum hose (don't forget to plug the connection to the inlet manifold) and driving slowly gingerly see whether the problem still exists
Chris

[roddy]

Might be worthwhile also including in the full check over, that the rear compensator is correctly adjusted.  The W/M gives details of settings.  An incorrectly set unit may have the rear brake callipers over or under working which can upset the braking balance and the general 'feel' of the brakes.

Regards -

[Richard Fridd]

Might be worthwhile also including in the full check over, that the rear compensator is correctly adjusted.  The W/M gives details of settings.  An incorrectly set unit may have the rear brake callipers over or under working which can upset the braking balance and the general 'feel' of the brakes.

Regards -

.    , if the seal is faulty the rear wheels may lock, or the lower the rear springs, through sag or lowering modification, the same effect. Shorten the rod to match lowered rear springs I think. Not exactly a feeling of a snatch though

[rogerelias]

To check servo, with engine switched off, pump the pedal until it goes rock hard, then with your foot on the brake pedal putting pressure on , start the engine as the engine turns over and starts the pedal should start to go down, if it doesn't then you may have a servo problem

[Jai Sharma]

Often in my experience the hose between manifold and servo is poor. An easy fix/cheap thing to try perhaps.
Otherwise I have found those small pistons in the front brakes sticky on occasion, although the fact they were rebuilt would tend to indicate that is less likely and your symptoms don't point to that.
It would be worth checking the pads slide nicely, as this is a simple thing to check too.
Good luck!

[lancialulu]

Was just looking at my sport running after successful mot and saw to my utter horror that the vacuum pipe was pulsing in and out. Seems like I fitted heater hose some 10 years ago when I restored the car. Seemed stiff enough at the time. Well as I had a spare length of 1/2 inch heater hose I replaced it on the basis I can do this every 10 years!! Brakes did not change behavious as the old hose was still pulling a vacuum. You need to check the one way valve is working too and also check there isnt any brake fluid in the m/c side of the servo indicating a leak of the rear seal......

[Dikappa]

probably not the cause in this case, but when you're at it do check/change the flexible hoses, as they often are the cause of locking up.  When they are blocked, pedal pressure will be enough to push fluid into the calipers, but is won't get back.  In fact they should be changed every 10 years....

[Caracad]

Thanks for all the advice.

I've been researching how brake servos work (I didn't really know).

--1.So with no brake pressure the vacuum from the engine acts on both sides of the servo diaphragm so no net suction.
      2.When the brakes are applied, pressure from the master cylinder opens an air valve which vents one side of the servo so that the vacuum only acts on one side forcing the servo piston rod to act on the brakes.
      3.On light braking the force is not supposed to be enough to open the air valve, so no servo assistance.

So I am thinking its an issue with the air valve on the servo which may be opening too soon.

I will investigate this when back home. Unfortunately this won't be for a couple of weeks as my partner wants me to go away and be sociable this weekend.

[fay66]

Sorry to sound smug as I have great sympathy for the problems servos seem to cause, but I'm just pleased that most series 1's didn't have them, neither have I ever felt the need for one.
Of course series 1 brakes have their own problems unknown to Series 2 and 3 owners


Brian
8227

[Jai Sharma]

I don't know if this is still the case but you used to be able to buy the non-return valve pretty cheaply e.g. from Omicron.

[davidwheeler]

Might be worth checking that the pads have not come off the backing - it has happened to me!

[Caracad]

Had a look at pictures and diagrams of the Fulvia Master Cylinder/Servo assy. and think I know what it might be.

There appears to be adjustment of the actuating Rod where it comes out of the servo.
I'm sure there is probably a specified clearance between end of rod and Master Cylinder piston.

As my car has had a new master cylinder it may be that there is too much clearance and so the servo is being activated before the master cylinder.

Anyone come across this adjustment?