Brake Imbalance - binding?

[Bryan]

I took my Bedford breakdown truck for MOT tonight and it failed again. This time there was a 66% difference in the rear brakes. I had already fitted a new handbrake cable, brake pipes and wheel cylinders. I had bled the system and adjusted them accordingly or so I thought.

I can't understand what the problem was other than the fact that there was a distinct smell of brake lining coming from the nearside rear wheel during the MOT test. This makes me think that the brake shoe was binding ever so slightly all the time and overheating causing the brake fade and thus the difference. When I came home I took the rear wheels of straight away and there was considerably more heat in the nearside rear brakes.

Has anyone ever seen this before? I don't want to slacken it off slightly just to go back and have the same problem.

 

[Daz500]

Even If you were binding, I'd still expect to get a pretty good brake when slamming on the anchors. What's the brake master cylinder like? Could possibly be a duff seal - you'd need to see how the front/rear split is achieved though.

Daft question but did the handbrake mechanism get reset properly when you did the cylinders?

 

[Gaz]

I took my Bedford breakdown truck for MOT tonight and it failed again. This time there was a 66% difference in the rear brakes. I had already fitted a new handbrake cable, brake pipes and wheel cylinders. I had bled the system and adjusted them accordingly or so I thought.

I can't understand what the problem was other than the fact that there was a distinct smell of brake lining coming from the nearside rear wheel during the MOT test. This makes me think that the brake shoe was binding ever so slightly all the time and overheating causing the brake fade and thus the difference. When I came home I took the rear wheels of straight away and there was considerably more heat in the nearside rear brakes.

Has anyone ever seen this before? I don't want to slacken it off slightly just to go back and have the same problem.

You could be thinking about it back to front. The smell of brake lining on one side could be because of the imbalance, not the cause behind it - one side is applying more pressure than the other.

I had a similar issue years ago, admittedly it was on a Civic and not a Bedford. Took multiple re-tests, new pads, new calipers, new brake hoses, new handbrake cable, and numerous other 'fixes' before I found it was all down to a little-known device which sits beside the master cylinder called a brake proportioning valve. It's responsible for apportioning braking pressure if your car weight balance is not normal, e.g. if you have a heavy load in the boot or towing something heavy. One of the brake lines into mine was clogged, so one of the rear brakes wasn't getting enough fluid pressure to it. Not sure if a Bedford would have one of these, but maybe worth checking.

 

[Bryan]

You could be thinking about it back to front. The smell of brake lining on one side could be because of the imbalance, not the cause behind it - one side is applying more pressure than the other

The smell of brake lining was coming from the side that wasn't braking as well according to the machine.

Worst about it is that it can't go back for a retest because it's about 6 weeks since its full test and it took this long just to find parts for the thing because no one makes any new parts for these old trucks.

 

[Coog]

Can't see the brake testing machine creating enough force to cause brake fade unless you ran it straight into the centre having had it out for a long drive? That would do it.

Either way you're going to have to sort the binding out.

 

[cormac81]

Just when you mention parts for these, there was a beford van/camper on a lorry today in Bleary, though I say it be heading for clearways there was bunch of oul mazdas on it too.

 

[FM155]

Sometimes the flexi's can collapse internally and give you an almost complete hydraulic lock. Brakes can't effectively work or return properly..

 

[Bryan]

Can't see the brake testing machine creating enough force to cause brake fade unless you ran it straight into the centre having had it out for a long drive? That would do it.

Well I drove from Waringstown to Portadown and then the truck didn't even get a chance to sit still before it was called in about 20 mins before the actual test time so the brakes would have been very hot if it had been binding the whole time.

I think I've discovered the problem - when I put the new wheel cylinder on the pipe twisted a bit with the connector on the end of it so I am guessing that the pipe may be slightly crushed or deformed inside limiting the flow in both directions and possibly adding to the binding problem. I'll stick a new pipe on it and see if that cures it. Other than that I can't find anything else wrong.

Flexi couldn't be an issue since the rear axle only has one flexi feeding the copper pipes that run to each side.

 

[FM155]

Thought that after I'd posted it, good to hear you've found it.