Brakes permanently on

Phil Robson

Phil Robson

Well-Known Member
I must apologise as this relates to my Alfa, but I thought you expert guys would be able to point me in the right direction. The principles are the same as a P6, although the Alfa set up is dual circuit, so has twin servos etc (both new).

IMG_3605.jpg

When I was out a week or two ago, I lost much of the brake pedal pressure & the brakes were only applying right at the bottom of pedal travel, with seemingly no servo assistance.

I have now bled the brakes & released a lot of air, which I thought would solve the problem. However, it made little perceptible difference when I reversed the car out of the garage. I then sat in it a while searching Alfa forums for an answer, then after about 20 minutes pulled the car forwards...or tried to. The brakes were fully on & I couldn't move the car. I had to disconnect the brake light pressure switch to turn the brake lights off last night!

When I disconnect the release valve's pipe into the back chamber in the servo, there's a hiss (loss of vacuum? - & only on one servo) but it doesn't make the brakes come off.

Does anyone have any ideas, please?

Apologies again for using this forum for a non-P6 query - I won't make a habit if it ;)
 
I assume that the servos are connected correctly? It does sound like you have a non-return refusing to release?

Try disconnecting both servos, to see if the brakes release..
 
chrisw said:
I assume that the servos are connected correctly? It does sound like you have a non-return refusing to release?

Try disconnecting both servos, to see if the brakes release..
Click to expand...

Thanks Chris,

I've run the car with great brakes for about 6 months so they are set up OK. I'll try disconnecting the servos though.

Of note is the fact that when you turn the engine on with foot on brake pedal, there's no change in feel.
 
chrisw said:
Try disconnecting both servos, to see if the brakes release..
Click to expand...

When the brakes were staying on today, I loosened the output pipe a little on the front servo & fluid squirted out under pressure. The wheels were then free to move.

This presumably suggests that the air valve in that servo is not releasing to let atmospheric pressure in behind the diaphragm?
 
sdibbers said:
Yep, sounds like the air valve is staying open which could be your problem.
Click to expand...
Yes - I think it's that way round - the valve is not shutting atmospheric pressure off when the foot pedal is released.

I've taken the servo air valve assembly apart & the actual air valve doesn't seem to be moving or is very stiff. I have one of the car's old servos & have removed the air valve & it seems to move much more freely:

IMG_3608.jpg

I've now used this old valve on the new servo & reinstalled it in the car. I'll bleed the brakes & see where we are :hmm:
 
That is a hellish environment for those servos to be in above the exhaust headers, I would not be surprised if you found things melted in there.
You may need to fit a heat shield, or ask your Alfa buddies if they are mounted correctly in the stock location.
The back corner of the battery looks like it may get toastie as well.
 
cobraboy said:
That is a hellish environment for those servos to be in above the exhaust headers, I would not be surprised if you found things melted in there.
You may need to fit a heat shield, or ask your Alfa buddies if they are mounted correctly in the stock location.
The back corner of the battery looks like it may get toastie as well.
Click to expand...
Indeed! Most cars just had one servo near the bulkhead for floor mounted pedals like mine, or one linked to the brake pedal for hanging pedals.

Certain markets had the pictured arrangements & I’ve never seen any heat shields……:oops:
 
last time I saw this it was caused by rust on the caliper pistons preventing them from retracting.
 
Just to wrap this one up, I couldn't find anything wrong with the servos or calipers (all new), so I replaced the only part that I hadn't done already - the master cylinder (under the floor) as I thought it could be sticking.

I remembered why I refurbished the old one with a service kit now - it cost £400 :oops: - but it looks to have cured the problem, so far......
 
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