Odd rear caliper problem

[neil w]

2003 A4 1.9 tdi 130 Avant, front and rear discs and pads changed 6 months and 6 months ago and all fine since then.

Got home the other day and the rear offiside (drivers ) side caliper was dragging badly and the discs was very hot.

When it cooled down I removed the wheel and expected the pads to be stuck in the caliper carrier or the carrier pins to be siezed.............. both were fine with no rust and moving freely and properly greased
Handbrake adjustment was fine and the handbrake cable is moving freely.

Caliper piston was a bit reluctant to wind back so I removed the caliper and bench stripped it , the piston was fine but there was very slight corrosion in the groove for the dust seal. The piston was polished and the groove for the dust seal and the piston seal grooves cleaned out and the caliper fully cleaned out .


The caliper was rebuilt using a set of new genuine seals that I had (very fine smear of rubber grease on the outer seal and fresh brake fluid on the inner seal ) , the caliper was definitely clean and corrosion free as the piston was wound back in by hand , everything built back up onto the car with new washers on the flexi pipe and bled through using a vacuum bleeder.

Brakes working fine with no leaks and releasing when checked statically as is the handbrake , however , after a 3 mile run the caliper is again red hot...... the handbrake is not siezed on and all of the other brakes are fine.

When left to cool down the brakes free off .

The only thing I can now think of is that the rear flexi is breaking down internally allowing fluid under pressure into the caliper but not allowing it to flow back when the brakes are released.


Any other ideas ????????

[calum]

2003 A4 1.9 tdi 130 Avant, front and rear discs and pads changed 6 months and 6 months ago and all fine since then.

Got home the other day and the rear offiside (drivers ) side caliper was dragging badly and the discs was very hot.

When it cooled down I removed the wheel and expected the pads to be stuck in the caliper carrier or the carrier pins to be siezed.............. both were fine with no rust and moving freely and properly greased
Handbrake adjustment was fine and the handbrake cable is moving freely.

Caliper piston was a bit reluctant to wind back so I removed the caliper and bench stripped it , the piston was fine but there was very slight corrosion in the groove for the dust seal. The piston was polished and the groove for the dust seal and the piston seal grooves cleaned out and the caliper fully cleaned out .


The caliper was rebuilt using a set of new genuine seals that I had (very fine smear of rubber grease on the outer seal and fresh brake fluid on the inner seal ) , the caliper was definitely clean and corrosion free as the piston was wound back in by hand , everything built back up onto the car with new washers on the flexi pipe and bled through using a vacuum bleeder.

Brakes working fine with no leaks and releasing when checked statically as is the handbrake , however , after a 3 mile run the caliper is again red hot...... the handbrake is not siezed on and all of the other brakes are fine.

When left to cool down the brakes free off .

The only thing I can now think of is that the rear flexi is breaking down internally allowing fluid under pressure into the caliper but not allowing it to flow back when the brakes are released.


Any other ideas ????????

A reasonable assumption and cheap enough to try, nothing to lose as you've addressed all other possibilities.


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[nevo]

Worth a try cheap enough,the tolerances on the piston to caliper are very tight so it looks like a new/secondhand caliper poss.

[neil w]

Well still no joy, bit the bullet and bought a brand new Lucas TRW Caliper and genuine rear flexi hose and swapped them , bleeding the system and setting the handbrake as per the Audi service manual and the brake is still binding.

All of the other calipers are fine, so pulled it apart again and wound back the piston (by hand ) but put it back together with the handbrake cable disconnected (to make sure the fault wasn't on the handbrake side).

Wheel spins fine until the footbrake is applied and then the wheel starts to bind so it must be an issue with the system not allowing fluid back from the caliper into the system .

Any ideas ?

[calum]

Well still no joy, bit the bullet and bought a brand new Lucas TRW Caliper and genuine rear flexi hose and swapped them , bleeding the system and setting the handbrake as per the Audi service manual and the brake is still binding.

All of the other calipers are fine, so pulled it apart again and wound back the piston (by hand ) but put it back together with the handbrake cable disconnected (to make sure the fault wasn't on the handbrake side).

Wheel spins fine until the footbrake is applied and then the wheel starts to bind so it must be an issue with the system not allowing fluid back from the caliper into the system .

Any ideas ?

Try a full brake fluid flush, there must be some crud in there somewhere.


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[neil w]

Full system flush already done and no faults showing up on the brake side on VCDS, can only think that it is a faulty body to axle flexi pipe, brake load sensing valve or a faulty master cylinder .

Really starting to bug me now

[calum]

Full system flush already done and no faults showing up on the brake side on VCDS, can only think that it is a faulty body to axle flexi pipe, brake load sensing valve or a faulty master cylinder .

Really starting to bug me now

I thought you had changed that, certainly sounded like the route you were taking, £10 part versus price of a caliper.


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[neil w]

Doh , changed the flexi between the caliper and the body but got it into my head that there was another flexi in the system ................... totally forgot that it has independant suspension and not a solid axle the same as my VW's which have a a second one.

[nevo]

Were the slider pins nice and free,failing that trace it back to the next joint and blow it through,after that its the abs unit !!!!,you could also remove the caliper fix the piston with a block or something similar and pump it up,see if it backs off.

[Rob69]

Iwas also thinking a problem with the abs block restricting back flow