mk1-forum.net

How do folks,

Right... The car has just been for an MOT and failed on the rear brakes.
It has a single line system with 4 pots on the front, a cooper s master cylinder and brand new rear plates with new wheel cylinders etc so everything is moving freely and isn't seized. There are no other valves on the system anywhere apart from a bias valve which is wound all the way out to allow for full use of the rear brakes. When the bias valve is wound all the way in, it cuts off even more of the rear brakes as we watched this on the brake test rollers.

The system has been bled from the furthest away from the master cylinder to the closest in order about 6 times.

The front brakes passed no problems at all and the tester said that the pedal feels good.

The rear shoes have been set so that they catch and then returned to where they are just free of the drum.

After chatting with Rich I fitted some smaller rear cylinders but this doesn't seemed to have fixed the problem.

Does anyone have any ideas of what else I can do? Its doing my head in.

What's is faied on?, was it little or no effort?, what readings did you get?

It failed on the back brakes and the hand brake being insufficient... I think it was at 40 when I was stamping on it rather than 100...

Rear brakes AND handbrake effort low doesn't sound like a hydraulic problem.

Are your shoes in correctly? Have you re-adjusted once the shoes have had chance to centre themselves? Where are the shoes from - I've had issues with Unipart ones before now being poorly sized.

If it's hand and foot brake are the shoes fitted the correct way round ?

I can understand if the handbrake readings are low which is probably due to the shoes not being bedded in yet.

If the foot brake problem persists you may need to shorten the coil spring in the limiter valve. It sounds as if the valve is too far closed,,,,,,,,,,,,maybe.

Rich has seen photos of the pads and they are not in the wrong way...

The shoes are mintex ones from minisport as they came pre built up. Do you think I should try some different shoes? If so what make?

I think the shoes have benn correctly centred etc.

Going to have another look when I have the time. I wondered if I should try and ditch the bias valve to see if it makes any difference but it must be working.

Graeme don't suppose you are passing by any time to take a butchers.

My thoughts are to try another standard equaliser or a T piece to prove a point.

Is the standard master cylinder up to working four pots and have enough left for the rear ?

If the fronts are generating enough torque then the rears should be working also . The master cylinder is generating pressure and since it is a single system then the rears will see the same pressure, so this does not seem to be an issue of insufficient fluid displacement from the master cylinder as you would be experiencing very long pedal travel if displacement form the master cylinder was the issue.

What pedal travel are you getting ... very short or very long?

Take the valve out and then see what torque is generated on the rollers if without the valve the torque goes up then the issue is in the valve itself but seems unlikely ..... my car passed with around 40 on the rollers .....

I'm going back to basics here, please forgive me if I'm stating the obvious, were the handbrake cables slackened off before adjusting the rear brakes? if the handbrake levers hold the shoes out, preventing them from pushing the pistons fully back into the cylinders, you lose a lot of hydraulic / mechanical advantage on both foot & hand brake.
New brake shoes take a long time to bed in on the back, that can drastically affect the readings.
I don't wish to be critical of any other person's advice, but why would you fit smaller cylinders? Larger cylinders would apply more hydraulic pressure to the shoes, but may increase pedal travel very slightly. I would think that 3/4" (19mm) bore cylinders, (GWC1102) would work well. Some years ago on my 'S' I had to replace the small bore cylinders with 3/4" for similar reasons, & the problem was cured.
I am not conversant with the type of brake bias valve you use, nor the rolling road you tester uses, but my current Mini reads around 65 which the tester says is the best he's ever seen on the back of a Mini.
I hope something here may be of help to you & good luck with your MOT.

Cheers for the replies folks.

There seems be be quite a lot of pedal travel which is why we fitted smaller cylinders.

I'm going to try and adjust them again with the hand brake fully backed off?

The superfins on the car came off my other mini which never gave me any issues

Handbrake adjustment will have no adverse effect as may have been described earlier . A hydraulic system acts like having a solid link between master cylinder and slave .... so wherever the pistons are in the slave the only thing that is affected is pedal travel not efficiency.

Either by pass the valve to prove if the issue lies there or puts hose clamps on both front hoses to isolate any issue of fluid consumption in the fronts.

WNX700H wrote:Handbrake adjustment will have no adverse effect as may have been described earlier . A hydraulic system acts like having a solid link between master cylinder and slave .... so wherever the pistons are in the slave the only thing that is affected is pedal travel not efficiency.

Either by pass the valve to prove if the issue lies there or puts hose clamps on both front hoses to isolate any issue of fluid consumption in the fronts.

Unfortunately I can't put hose clamps on because the hoses are braided...

I'd only be expecting to see between 60-80 on the rears (foot brake) and a locking handbrake, if the handbrake is low have you got another set of shoes you can try as the new ones may not have had a chance to bed in yet

Tom, unfortunately I'm north more than I'm south these days so won't be passing for a week or two (last week would've been great, could've done with something to ease the pain of the M6 being broken!)

What adjustable bias valve do you have? I know there are some out there that are never 'fully open' and will also provide some restriction.
That said, I'm not convinced it is a hydraulic problem otherwise I would expect the handbrake to adjust up and work OK. So, the common links between footbrake and handbrake is in the drum - whip them off, take some photos and post them up, lets see if we can spot an obvious oversight.

Tom, I doubt the rear shoes have bedded in yet it's about 2miles from your workshop to the mot centre. They have perhaps centralised more not and need adjusting a bit more, so rack the adjustment up a bit more and see how it feels, failing that pull your hand brake when he's not looking Haha!
Your front brakes will be spot on as captain moody will have bedded them in for you when it was on the rollers..
It's a shame he wouldn't let it go at that.. As I imagine they would be just about perfect at the moment for what you need it for (up hill climbing)

Its knackered now until I get a new front hose anyway I will revisit it next week

Well, it has now passed it's MOT which is good, but I am still not happy with how much travel the brake pedal has. When I stamp on it I can press it all the way to the floor nearly.

Any suggestions? Or should I try and bed them in for a while?

Are they better with a notch or two pulled on the hand brake ?, if they are might be worth giving the rears some more adjustment, if not then you could try lifting the front of the car then leaving it on axel stands with some pressure applied to the pedal overnight, you may still have some air in a pipe as its all new, sometimes this can get it out, or show if you have a slight leak in the system somewhere

I'll try them with the hand brake pulled tomorrow and let you know. Next step is try a pressurised bleeder...