NEW SERVO AND STILL ALL LOCKING ON!!

H

heapy

Member
Hello can anyone tell me why my brakes are all still locking on?? the old servo gave up the ghost recently by locking all the wheels up and i could only get moving again by unscrewing the "out" brake pipe at the piston end of the servo (releasing the pressure) upon inspection the servo was full of fluid too and it was just cheaper to buy a brand new whole servo instead of paying the rip off prices for 3 bits of rubber to fix the old!
anyway after plumbing it in, i didn't bleed it fully and took it for a quick test drive and the brakes all locked on again.
i had to release the pressure at the end of the new servo to get going like before.. so i then bled the system properly and all seemed to be fine until tonight on the m25 they all just locked up again in the middle lane and i managed to pull over in a cloud of smoke from the brakes.... did the usual and all is fine again???!!
The calipers front and rear are all fairly recent, it cant be a rubber pipe collapsing as that would be just one wheel and releasing the pressure at the end of the servo wouldn't make any difference if it was that. The master cylinder i had recently replaced the rubbers in too though it was a little pitted but i don't think that could place pressure through the servo like this?? could it? its certainly not leaking.
so any suggestions? Cheers :O)
 
I had locking brakes when i rebuilt my master cylinder, and was stupid enough to fit a chamfered washer at the end of the cylinder the wrong way around. Therefore, the piston would not retract fully and it would not allow the fluid to return.
Perhaps it is worth checking.
Or that you 've got the pedal height and travel correct.
 
Hello Heapy,

Did you build the new servo yourself or was it an exchange unit?

It sounds very much to me as if the air control piston is not retracting following a brake application, and as such the brakes will lock on and not release.

I'll wait till I hear back from you.

Ron.
 
hello, no i got the seals correct in the master cyl lol. :O) been in there since the beginneing of the year. though the bore was pitted. i thought id stick the seals in there and try the cheapo option first, if not buy the whole cylinder and they were/have been fine, well working since without leaking anyway.. i was wondering if the seals were breaking down from the pitting perhaps and somehow buggering the servo up but i cant see how that could effect it ie keeping a constant pressure there without my foot???? can any body shed some light on that theory?

As for the servo i bought a new one and didnt touch it, just plumbed it straight in. its not a genuine lockheed i noticed if that should make any difference, though an exact copie and in the applications it says 2.2 p6. i was wondering if the servo could be duff though i would have thought this to be highly unlikely and more like something else, plus i dont want to look a wally if i take it back !!. one min everythings working nicely and the next the pedal starts to go rock hard and all the dramas start and the only way is to release the pressure at the end of the servo and give every thing a good whack which is exactly what was going on with the old one though alot less with the new one..
what you think?? :O)
Cheers, Keith.
 
I am just wondering if your master cylinder is sticking.

I knew someone whose clutch would stick open now and then and it was the clutch master cylinder stcking down. It only happened when he reversed out of his parking space at work though. It took a while, but we worked out that as he was spun round in his seat he pushed the clutch down a bit further into a place where it had rusted slightly, and it stuck.

Next time it happens, try tapping the master cylinder.

Richard
 
I agreee with them.

Three possibilities. Air admission valve on the servo (the white platic lump a front top), fotbrake adjusted incorectly so that the master cylinder can't return far enough, or master cylinder defective giving the same effect.

Chris
 
ok, i suppose it will be the actuall tiny piston below the valve sticking and not the small diaphram etc under the plastic cap so therefore no use to change the upper part back to the old.?? the old tiny piston was knackered anyway too. if it is that?

Righto i'll do what i thought. i'll buy yet another new part ! lol.. i'll have to get a new master cylinder from wadhams and try that as its more likely than a duff new servo. as for the pedal adjustment prob not as all was fine before though i'll still double check.
Cheers, thanks and ouch my wallet... :)
 
When the brakes lock on, and you slacken the outlet pipe from the servo to release them, that only proves that it is either the servo or the master cylinder that is causing the problem. Next time, slacken the inlet pipe to the servo, if the brakes free off, it's a faulty m/cyl, if they don't, it's the servo.
 
Is there a non return valve where the vacuum pipe joins the manifold ? YEARS ago I was having servo trouble on a Ford Corsair and the valve was getting too hot and sticking
 
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