Help needed: Odd Brake Forces

[Fredjohn]

I mentioned what follows in the MOT pass/fail thread, but thought it might get buried by other MOT posts.

Although my car passed it's MOT today after failing on Thursday, the various brake forces of the pass/fail results seem most odd.

I stripped down one proportioning valve(rear UK driver's side), cleaning it and then replacing/bleeding that corner. I didn't touch any other caliper/valve/MC at all.

But the fail and pass brake test results are baffling to say the least (it failed on the rear axle balance L to R):

Fail results:
Front L 283 HB L 155 Rear L 148
Front R 389 HB R 156 Rear R 101

Pass results
Front L 313 HB L 113 Rear L 75
Front R 303 HB R 156 Rear R 98

The really strange one is the hand brake. I didn't change or adjust it in any way, so how come the reading drops from 155 to 113? Similarly the front brakes were untouched.

Could it be that the pads weren't warm enough? Could the fluid be off and need totally replacing? Or is there some other aspect I have overlooked?

Edited to correct brake corner labels. Thanks Simon see below.

[Simon_P]

It's not clear which is the rear right - I'm not being dim, they are paired diagonally, so I'll guess the bottom one is right.

In the fail set one circuit is working more than the other. Suggests a sticking piston in the master cylinder (car needs more use)

The thing that baffles me is why you cleaned the proportioning valve?
And then only one? There is now a c. 20% difference in proportioning circuit to circuit.

The handbrake is a relatively simple and ineffective mechanical system. It isn't a science, It only has to work. I wouldn't read anything into the results.. Could be the tester, could be the weather.

My advice is to drive it more and think about it less!

Have fun.

Oh, and you should change the brake fluid every 2 years.

[Fredjohn]

Thanks Simon your guess was correct. Have edited post accordingly.
I stripped and cleaned both valves 3 years ago, so I only cleaned one valve this time because it was that corner that was "faulty" and I thought it may be blocked by crud. It was.
I'll flush through a load of new fluid as soon as I can get the car back up on the lift (currently occupied by an exhaustless S4 Elan).

[alan e]

How I would love I lift in my garage the roof is to low for one