Rear brakes not good enough

[gr4h4m]

Oh and at some point I changed the brake pedal to a disc brake pedal from the older drum pedal. Not sure that made much difference but I was trying to get the best pedal possible

[sclemow]

The weight of the car is interesting, I hadn't thought about that.

To be clear, I'm not worried about the braking performance, as has been mentioned a few times, they don't need a lot at the back. I do like to keep it MOT'd though and want to get it through the test, this is where I am struggling at the moment,

[sclemow]

Does anyone know off hand if the adjustable rear bias valves can be adjusted while it's fitted to the car?
It looks like the adjuster would be up against the rear frame.

[GraemeC]

Working from memory (ie not going to have a look!) I'd say the adjusted would be innaccesible if the valve is mounted in the standard way (in fact it may not even fit?)

[sclemow]

hmm, I suspected that may be the case, which is going to make it a pain to set up!

[Peter Laidler]

This is what I was taught about vehicle handbrakes. Mind you, we did have some vehicles with ratchet handbrakes. But straight direct cable was this.
1st click wheels to rotate freely
2nd click, to bind but able to rotate with a BIT of effort 'a BIT' was left to the bloke doing the adjusting..... But the effort was to be the same on both sides
3rd click Wheels locked on.

Just my very limited experience, as taught in the classroom and on the workshop floor of vehicle engineering.

As for the bias adjuster, I hope others will chime in, for or against, that for a road car, just stick to what you've got

[sclemow]

Thanks Peter. The handbrake is fine, it is all working and passes MOT requirements. The footbrake is not providing sufficient efficiency for the MOT, so something needs to be done to the brake system as a whole to address it.

['S'-type]

This is obviously a really old thread, but I was wondering if you found the problem?
I am having similar problems with my S. The handbrake is ok, brakes are adjusted with new cylinders etc, and are bled correctly.
The shoes have been on for a couple of years. It has always been marginal on the rear brake effort on the footbrake, and is below what it needs to be.
I have the standard rear bias valve, S discs on the front, with no servo and the smaller early GT wheel cylinders.
The brakes feel fine on the road, but I don't think you'd notice underperforming brakes on the rear.
I tried warming the brakes thoroughly and putting it back on the rollers, but this evened the brakes from side to side but didn't affect the efficiency.

Any ideas?

I think warming the drums is a red herring. As they heat up drums expand away from the linings, however slightly, so in use they tend to 'go off' more quickly with heavy braking. Not your issue here of course.
My only other thought is some kind of restriction in the rear lines, apart from the bias valve itself. Are the rear flexis fairly new? I had a problem with a front flexi once. It was letting fluid through (ie when bleeding) but it seemed to require more pressure to reach the cylinder on that side and when it did it grabbed that side of the car. So I had the weird situation of the car grabbing first one side then suddenly pulling to the other.

Sorry don't know about in situ adjustment of valve.

[Exminiman]

I know its an obvious one but, have you measured the inside diameter of the drums in case they have been skimmed or skimmed too much. Also the condition and width of any wear on the drums can effect where they make contact (or not) .

[Polarsilver]

My thoughts are providing the brake shoes have the leading shoe the correct way around.. Handbrake is as you say good + flex Hoses are OK & pass fluid ?..the next thing is take off the rear subframe mounted Valve open it up & it could well be internal corroded or the seal is knackered.. if its beyond saving buy a replacement standard valve from MS & hopefully the problem is sorted... if not any of those then i use my early mk1 genuine " Sooty Magic Wand" to sort all other issues.

['S'-type]

My thoughts are providing the brake shoes have the leading shoe the correct way around.. Handbrake is as you say good + flex Hoses are OK & pass fluid ?..the next thing is take off the rear subframe mounted Valve open it up & it could well be internal corroded or the seal is knackered.. if its beyond saving buy a replacement standard valve from MS & hopefully the problem is sorted... if not any of those then i use my early mk1 genuine " Sooty Magic Wand" to sort all other issues.

That valve is the likely culprit. If the hand brake can meet the test level, then equalised hydraulic pressure at all four corners should be enough to do the same. So what is over-restricting it to the rear?

As an aside, we didn't have this problem when the test stations didn't have rollers. They just used to drive round the block with a decelerometer (Tapley) in the footwell while you waited nervously for the verdict. They are still used as a back up.

Still it's njce to know what the rears are doing.

[sclemow]

I agree that the valve is the most likely candidate.

I have put the S through with a Tapley device before, but keep coming up against this issue so am determined to give it a good go at fixing.
After stripping and taking a good look at the bias valve, I am thinking that packing the spring with washers should stop the valve closing so early, and allow more fluid into the rear system, giving more braking effort. I don't know whether I have an S bias valve on it or a normal one, as over the years I've struggled to find out what the difference is in the spring and how you tell, but it seems to me this should improve things.

[imack]

What sort of braking effort does the mot station say you are currently getting on the rear wheels?
As an ex mot tester who used to test minis on a regular basis back in the 80' and 90's, I wouldn't expect to see much more than about a 50kg braking effort from each rear brake on a mini with single circuit brakes even when everythingis perfectlyand correctlysetup. The hand brake should lock the wheels and lift the car out of the rollers, the foot brake won't do this on the rear wheels though. Later dual circuit minis with the inertia valve on the rear may raise a little more on the rear foot brake on the roller brake tester.
How experienced is the mot tester with classic cars and early minis?
This is one reason why historic cars became exempt from mots as younger/inexperienced testers were incorrectly failing classic cars.

[sclemow]

This is an MOT station with an experienced tester, who does understand and own a number of classics. It's a very good point though, and one that I try to avoid running across. They are in fact the ones that put it through with a Tapley device before, and this is an option if I need to.
They have new rollers that automatically measure the weight of the car and set the required braking effort accordingly. It's registering about 35% and they need 55% plus so it's quite a way off.

['S'-type]

I agree that the valve is the most likely candidate.

I have put the S through with a Tapley device before, but keep coming up against this issue so am determined to give it a good go at fixing.
After stripping and taking a good look at the bias valve, I am thinking that packing the spring with washers should stop the valve closing so early, and allow more fluid into the rear system, giving more braking effort. I don't know whether I have an S bias valve on it or a normal one, as over the years I've struggled to find out what the difference is in the spring and how you tell, but it seems to me this should improve things.

Yes shortening the spring was the way to reduce the pressure required to close the valve so by that logic doing the opposite should help. But it's a bit of a trial and error thing. (ie tedious)
If it's for road use would it be worth trying another one altogether to see if it changes things. All mk1,mk2 cars inc Cooper and S and the mk 3 S shared the same valve.

[mk1coopers]

35% efficiency for the whole system, or just 35% contribution to the overall efficiency ?, I'd also only be expecting to see around 50kg of force from each side on a single circuit system, into the 60's if it was exceptional.

I have the same issue with my French 1980's hot hatch, even though that has rear discs, they are tiny and the pads are only around 7mm thick when new, even now, with properly bedded in new discs and pads, new subframe valve, new rubber hoses, new hard brake lines all the way through, and new calipers, it struggles to get above 80kg, it has always been the same for the 30 plus years I've had it.

[Ronnie]

If you are willing to strip and adjust the limiting v/v, why not put a pressure gauge in the circuit and check the pressure in the rear circuit see what it is.

[imack]

Overall braking efficiency for that age of car needs to be 50%, (58% on modern cars) I don't know what the 55% figure is they are referring to.
What overall efficiency does it record on the tapley meter? It will still need to be over 50% to pass.
Do you know the values recorded on each individual wheel on the roller brake tester?
Do the front wheels lock out? (The rear wheels won't lock out on a mini, although they will on modern cars)
Is the braking effort even across the axles?

[Catmint]

Going back to an earlier post, you mention mintex shoes - they do/did make a competion compound (which MS sell or did do)and if these are fitted then they need bedding in by getting hot - if not bedded in they do not work and give a numb pedal/ handbrake feeling with very little stopping power. Might be an idea to try a set of different shoes in or drive around for a while with the hand brake on!

[sclemow]

Thanks Catmint. I think that might be someone else using mintex shoes. Mine are new standard shoes, but I did wonder if the material isn’t as good as the old nos ones. And as people have mentioned whether they are the same size.