Rear brakes sticking on

[pabr639]

Me again, 996 C2 1998 3.4 Tiptronoc. This has been a massive project, I bought the after being stood in a a yard for 4 years, To many jobs to list, But I have replaced all the brake pipes, a new master cylinder. And the braking system seemed fine, Took it for MOT today, The brakes where fine on the tester. but when I wend to drive home the rear calipers where locked on, I thought it was the hand brake stuck on, When they cooled down they will release , But when I eventually got the car home and got it up on the ramp, the brakes where now free, Then i pressed the brake pedal hard and they jammed on again, but not the fronts, A lot on the forum about brake booster, but surly that would be the back and fronts. this is just the rears, I am thinking there has to me some thing between the the master cylinder and the rear calipers, Anyone have any ideas?.


Paul

 

[Alex]

ABS?

 

[pabr639]

abs ? Would it throw a fault code?.

 

[deMort]

Are you sure the brake master cyl you replaced is the correct one for your car ??

You press the brake pedal and the rear brakes stick on .. that's either calipers .. X2 .. hmm .. handbrake .. but you haven't used this .. btw you tighten the handbrake adjuster till the disc won't move then back off the " wheel " 3 turns of a screwdriver ... check this please .

Or the master cylinder and more so the rod is too long and allowing some brake pressure to retain .. i would expect x1 front and rear to be sticking in this case as it's a diagonal system but without seeing the car it's hard to say .

Calipers or master cyl is all i have as an answer .

fault code test on the abs might be useful .. never seen it before but the abs can apply or release the pressure to the brakes ..

As Alex said .. ABS but the speed sensors causing the issue ... it would have to be both though .

I'm struggling to see this as the fault but a fault code check is the first point of call .

 

[pabr639]

Ok, I will do a scan in the morning, There cannot be many things that will lock the rear calipers , It is definitely not the handbrake. It was fine for a while after changing the brake pipes a bleeding the system.

 

[pabr639]

Just another pointer, When the car was cold on the ramp, I pressed the brake pedal hard and both rear brakes locked, so I thought I would take the pads out to see if I could see the problem , Like are the pistons seized, The only way I could get them out was to undo the bleed nipple to release the pressure, When I had released the pressure from the bleed nipple the pads came out fine and the pistons where free. Looking on the forum last night I noticed I should have anti vibration dampers on them and I don't have them, But I cant see how this could cause my problem. A head scratcher, Back in the garage shortly.

Paul

 

[Griffter]

Did you replace the rubber flexis? These can breakdown internally and act like a one way valve. Otherwise another obstruction somewhere in the hydraulics might do the same thing? Even a pinched metal pipe can expand to let the fluid through under pedal pressure but not allow it to return.
I'd start by bleeding all the fluid and inspecting every inch of the pipes.

 

[Alex]

The pressure applied to the pads vs the relief of pressure once the pedal is lifted are massively different so as Grifter states, pinched pipes of degraded flexis would cause it.

 

[pabr639]

Good morning, Just started to try to find this problem, I have just undone the bleed nipples to one side of the car calliper and pushed the pistons back so I can put the pads back in. so both wheels now free, I have applied the brakes quite hard , No ignition and no engine running, And the wheels locked solid, my process of eliminations, I broke the pipe connections to the master cylinder, No change rear wheels still locked, I then broke the feeds to the abs unit and no change, and then the pipes from the abs to the rear, and as soon as i cracked the not, I immediately heard the rear brakes release. Both wheels free now, No codes in the abs module, So ABS unit faulty????


Paul

 

[Alex]

Sounds like that could be it.

 

[pabr639]

I think so, I will let you know how I get on, Thanks for the help.


Paul

 

[pabr639]

Next question is , Can I buy a used ABS unit and just swop it over? I know some ABS units have to be programmed.

 

[deMort]

This is a bit of a strange one as technically this can't happen .. you would certainly expect a fault code .

Repeat the test but with the battery disconnected please .

Abs unit on these has a little coding for which model it is but can basically be fitted with no coding to prove your issue .. if bought from a C2 then the coding shouldn't need altering .



Something is bugging me about this though and i can't put my finger on it .. C4 and you can get a master cyl failure that puts too much pressure into the abs unit so that it can't release the pressure .. but you have a C2 ..

I know this as i spent days trying to figure out that issue when the master cylinder was a new one .. turned out to be faulty ( actually not technically correct .. it broke due to factors we didn't realise at the time )... this was a C4 though ..


Ok .. does this do the same locking of the rear brakes with light brake pedal use ? maybe light press the brakes a few times .. see if its a build up of pressure , or if it's only hard braking that does it .

Logically it's the ABS unit .. but i'm just not sure atm , a doner unit would obviously prove this and hopefully fix the issue ... But some things don't sit right with me and this is one of them .

EDIT ..

Ok this is really bugging me and made me think about this issue ..

You drove to the mot station with no issues , did at any point you go around roundabouts on the route there and did the PSM light ever flick on to indicate the psm might have kicked in ? think hard about this please .

At the mot test they would have applied FULL braking .. far more than you will get in a road situation and basically the issue started after this ..

have you done a steering wheel calibration of the angle sensor at all ? can you check this , in other words do you have a tester that can see the angle of the steering with wheels straight ahead ?

ABS unit .. yup its possible .. i guess .. but i'm just not feeling that atm ..

I have no real way of proving or disproving this fault , checks mentioned might help but to me its either the ABS unit or my feeling .. the brake master cylinder thats at fault ..

This is for me an itch that can't be scratched .. hopefully it's the ABS unit but im unsure .

 

[pabr639]

Hi And thanks for your help.

No I have not done a steering wheel calibration, I can try later

I have just pressed the brakes with the battery disconnected and they are now locked solid and they where free before.
The only way to free them now is to release the pressure between the ABS and the calipers, The ABS unit is 100% holding the pressure and not releasing it.


When I drove to the MOT station with no problems, no warning lights whatsoever .

A bit of history

After changing all the brake lines, I had a lot of trouble trying to beled the brakes, So I fitted a new Master cylinder, Then I could only get fluid through to the fronts, I then got a mate to pump the pedal fast with the rear nipples open, and that got the fluid flowing, then the the brakes seemed fine,

I really appreciate your help,

I will be amazed if it is not the ABS unit,, """"""But I have been wrong before"""""""



Paul

 

[Griffter]

It sounds like a physical (as opposed to electrical) fault with the abs unit.
The car was standing for 4 years so dirt or corrosion could be in there.

Alternatively it sounds like the abs unit was emptied of fluid then you tried to get the fluid into it and bleed it by pumping the pedal. I've never done it (always try to ensure the abs unit does not run dry) but my understanding was that you need to get the fluid through the abs unit with a pressure bleeder and/or PIWIS.