Brake Problems!

[Wesley]

On Monday I was driving down the road and I felt that the car was pulling back. A few minutes later it stopped. I then realised that all 4 brakes where locked on solid.

 

Now if I pull the servo vacuum pipe off it releases the wheels. If you then put the servo pipe back on you can drive for a few minutes again and it then locks up again.

 

Has anyone else had this problem. I have spoke to 12 different mechanics from small garages all the way up to fords and no one has ever had this problem.

[sparky Paul]

Sounds like a fault within the servo, possibly a faulty seal allowing air into the input side of the diaphragm.

[Wesley]

I changed the servo and master cylinder. That didn't work so I changed the servo vac pipe and that never worked. So I changed the vac pump and still the same.

[insanitybeard]

Was it a new genuine or secondhand/aftermarket servo unit you fitted? If you've changed all that then there can't be much left to look at. A faulty servo vacuum feed pipe wouldn't cause the brakes to lock on, nor a faulty vac pump. Edited September 13, 2014 by insanitybeard

[Wesley]

I took it of a working car that had gearbox problems. I only drove it a few days ago so I know the brakes where spot on.

 

From what I can tell the servo is getting too much vacuum that's why I tried the pump and also the pipe with the none return valve. (They where both suggested by fords to)

[sparky Paul]

Ah... further information. ;)

 

Excessive vacuum shouldn't cause this. When the pedal is released, the input side is vented via the bypass so that both sides of the diaphragm are at the same vacuum - hence no assistance. If it's servo related, either air is getting into the input side, or the actuator is not being released far enough to open the bypass.

 

When you changed the servo/master cylinder, was any of the adjustments loose, in particular on the rod to the pedal assembly?

[Wesley]

No all seamed fine

[Wesley]

I have drilled a small hole in my spare vac pipe as it comes out the servo releasing some pressure.

 

Brakes no longer lock up and works as they did before the problem

 

 

Obviously this doesn't sort my fault

[sparky Paul]

It's certainly a strange one this.

 

If reducing the vacuum makes the brakes operate normally, it still points towards a fault within the servo, or maybe a sticky master cylinder... but you have changed both servo and master cylinder. Any fault within the ABS block shouldn't be affected by releasing the vacuum, likewise collapsing hoses etc..

 

When the brakes lock up, does the pedal fully retract when taking your foot off? Have you tried the the old 'toe under the pedal and lift' trick?

 

Have you done the usual simple servo tests, and did it show up anything unusual?

 

Vent all vacuum with engine off, depress pedal several times to exhaust vacuum

Start engine with foot on pedal, pedal should give way as vacuum assistance builds up

Switch off engine. Subsequent depressions of the pedal should get harder, all vacuum should be exhausted after several pushes.

 

 

[Wesley]

When they lock up the pedal is at the top just hard.

 

Checked all that and it seems normal.

[insanitybeard]

Any chance of any wear or damage to the pedal box bushes or brake pedal/linkage to the servo rod preventing the rod from sitting in the correct position when at rest (therefore allowing slow release of vacuum into the servo- though I don't know how far the rod has to be depresesed before this would happen- more than would be allowed by a little pedal box wear I suspect)? There is also a square seal on the reverse of the servo unit, I'm not sure if this affects anything but was that renewed when you substituted the servo? Did the substitute servo have the same part number as the old one? Edited September 14, 2014 by insanitybeard