17 replies to this topic

[Davebullock]

Hi,

My mini 1975 clubman just failed MOT on back brakes.

With an assistant pressing the brake pedal I can just manage rotate the back wheels (with great effort).

 

I checked the rear shoes and they are OK

I checked the pistons in the rear cylinders and they all move freely.

So initially it seems like either there is a partial blockage or the pressure limiter valve is the culprit?

 

However... both wheels brake in exactly the same way with equal force needed to turn the wheels, so if its a blockage then it would suggest that the fault is in a common component.

But the brakes are single circuit and the only common pipe is BEFORE the limiter valve.

Secondly I would expect that if the limiter valve were leaking internally (piston seals gone) then the braking would be too great not weak?

 

Does anyone know:-

What pressure does the brake limiter shut off the rear braking at?

One should be able to fit a pressure gauge in one of the bleed nipple outlets and measure it?

 

How can one test the brake limiter valve?

 

Anyone had a similar problem and found the solution?

Please enlighten me...thanks in anticipation

Dave

[sledgehammer]

my memory isn't great - so this will need verifying by others

 

the valve (compensator) you are taking about can stick in the restricted position if you bleed too fast

 

I have in the past tapped them & re-bleed them slowly

 

it basically restricts the flow as the back is quite light & locks up

 

this is my memory of the valve - bolted to the rear subframe - haven't worked on one for years - so check with others

 

IIRC most later ones had a different valve on the bulkhead - dual circuit

 

I think this is the valve - https://www.minispar...|Back to search

Edited by sledgehammer, 26 May 2018 - 07:15 PM.

[carbon]

Dave, has any work been done on the back brakes recently, and have they been adjusted up tight (almost binding)?

 

Could just possibly be brake rear pressure limiter. Not aware of any easy tests, substitution for a known working unit may be your best option.

 

They are also possible to refurbish if you are confident in your abilities.

[Spider]

What braking set up you have (ie, 8.4" or 7.5" discs or drums on the front - yes - this is relevant) and have you checked what dia rear wheel cylinders are fitted? The rear shoes also have a leading and trailing edge and it's important that they are fitted the right way around - yes - it is possible to reverse fit them!

 

I feel from your description that they are only just a little light on, however, they should never lock.

 

There were are seven different pressures the rear mounted valves shut off at depending on what the car was originally built with, from around 220 PSI up to 450 PSI. There are after market adjustable types available for these, though I'm uncertain how the MOT man might feel about them.

 

You can increase the line pressure at which it shuts off easy enough by fitting a thin washer under the spring in it, by thin, a washer of 0.25 mm thick will make a big difference. If you do this, check on a quiet wet road that the rears don't lock. If they do this, it'll swap ends very quick on you under brakes and it won't need much braking to do so.

 

The only way to test it really is as you are thinking, ie with a pressure gauge, but really, you need two gauges, one fitted before the valve and one after. The Gauge will need to be rated to a minimum of 1000 PSI but don't drive anywhere with a gauge fitted !!

[weef]

Dave, if you suspect the limiter valve performance to be faulty and not the wheel set up, pipe out the valve and see what the difference is and then repair accordingly.

Also check the drum diameter is still within spec, the flex hoses and that there is no contamination of the shoe linings.

[timmy850]

There's a helpful video here on the orientation of the shoes and adjustment as well. The adjustment does make a huge difference to the brake performance (especially when you have 4 wheel drums like me)

https://youtu.be/M02bqqNOlkY

[nicklouse]

you can modify your existing regulator to give more pressure to the rear cylinders.

[whistler]

What braking set up you have (ie, 8.4" or 7.5" discs or drums on the front - yes - this is relevant) and have you checked what dia rear wheel cylinders are fitted? The rear shoes also have a leading and trailing edge and it's important that they are fitted the right way around - yes - it is possible to reverse fit them!
 
I feel from your description that they are only just a little light on, however, they should never lock.
 
There were are seven different pressures the rear mounted valves shut off at depending on what the car was originally built with, from around 220 PSI up to 450 PSI. There are after market adjustable types available for these, though I'm uncertain how the MOT man might feel about them.
 
You can increase the line pressure at which it shuts off easy enough by fitting a thin washer under the spring in it, by thin, a washer of 0.25 mm thick will make a big difference. If you do this, check on a quiet wet road that the rears don't lock. If they do this, it'll swap ends very quick on you under brakes and it won't need much braking to do so.
 
The only way to test it really is as you are thinking, ie with a pressure gauge, but really, you need two gauges, one fitted before the valve and one after. The Gauge will need to be rated to a minimum of 1000 PSI but don't drive anywhere with a gauge fitted !!

I've just replaced the rear shoes on my wife's '84 998 and both sides were on the wrong way round. leading edges together. Worthwhile checking. Also during the 70's I believe, there was a change of wheel cylinder piston diameter.

[absx2]

Hi,

My mini 1975 clubman just failed MOT on back brakes.

With an assistant pressing the brake pedal I can just manage rotate the back wheels (with great effort).

 

 

Sounds just right to me. 

Is the MOT station taking the vehicle weight and age into account eg. no inertia limiter valve or load compensator ? 

 

The very last thing you want is the car swapping ends due to too much braking force on the rear. 

I use an MS72 adjustable valve on everything and the rear wheels can be turned by hand ( big hands ) and have never had an MOT fail so I think it would be a good idea to politely ask the tester to check the manual before going to far with the rear brakes as it may make your car dangerous in an emergency stop or just wet weather. 

[Davebullock]

HI everyone,

Many thanks for all your thoughts. Sorry for not replying earlier I thought I would get an email saying I had replies but I didn't.

Is this a feature one has to turn on?

Anyway....

I have 8.4 discs on the front and 12" wheels and if I remember rightly 3/4 bore cylinders.

However the rear brakes have easily passed previous MOT's. So this is a recent fault not due to any mechanical changes that I have just made.

With an assistant gently ramping up the footbrake pedal until it is hard... I can still (just) revolve the rear wheels with great effort.

Consequently the brakes didn't even register on the rolling road and so failed!

The brake shoes are UN-contaminated and the correct way round and the adjusters one (or two) stops from locking the wheels

Both exhibit exactly the same amount of poor (foot) braking. ie both wheels can just be rotated. Whereas the handbrake throws the car out of the rollers!

I was just concerned as the fault seem common to both wheels which seemed to rule out anything beyond the limiter valve.

Yet there might still have been something I hadn't thought of ... hence my inquiry.

Thanks

Dave

[absx2]

Quote from Moke Spider.

 

I feel from your description that they are only just a little light on, however, they should never lock.

 

You can increase the line pressure at which it shuts off easy enough by fitting a thin washer under the spring in it, by thin, a washer of 0.25 mm thick will make a big difference. If you do this, check on a quiet wet road that the rears don't lock. If they do this, it'll swap ends very quick on you under brakes and it won't need much braking to do so.

 

I would take Mr spiders wise words and fit a thin washer.

It seems very strange that you can just manage to turn the wheels but they don`t register on the rollers. Have you not been having your Weetabix   

Edited by absx2, 28 May 2018 - 04:12 PM.

[cal844]

Move the adjusters slightly tighter.

I ask mOT man to do a mock test one month before the test date and get a list of failures and advisories

[carbon]

The combination of 8.4 inch discs on front and 3/4 inch rear cylinders sounds like a potential recipe for disaster if you still have the original 1975 rear brake pressure limiting valve fitted on the rear subframe.

 

From experience with 7.5 inch discs on front and 5/8 inch rear cylinders you need to be very careful that the original type rear brake pressure limiting does not cause the rear wheels to lock up under heavy braking.

[timmy850]

The 8.4" discs and 3/4" rear cylinders were a factory combination with the master and limiting valve below:
Master cylinder GMC227
Limiting valve FAM7821

There is a list of the factory combinations here:
http://www.minispare...rt.aspx?1~9~105

Edited by timmy850, 30 May 2018 - 12:07 AM.

[Spider]

The 8.4" discs and 3/4" rear cylinders were a factory combination with the master and limiting valve below:
Master cylinder GMC227
Limiting valve FAM7821
 

 

Yes, however, the OPs Brake system is Single Circuit with a 21A1201 (or similar) Pressure Limiting Valve.

 

 

 

HI everyone,

.

I have 8.4 discs on the front and 12" wheels and if I remember rightly 3/4 bore cylinders.

 

There is something not right there at all. The rear brakes should be just about locking up with 3/4" cylinders, however, as Carbon touched on, this is a very bad combination as with everything working as they should, the rears should be pretty much locking up with little more than the shadow of your foot over the brake pedal.

 

I'm just wondering if in fact the Pressure Limiting Valve has been previously tinkered with to shut off at a very low pressure?