30 replies to this topic

[TopCatCustom]

Is this a road car? Or a car that has to see road use? Having the brake bias adjustable from the driver's seat is very much illegal.

It's not for road use. Why on earth is fine adjustment illegal when people on this thread are talking about vastly changing the size of wheel cylinders which could have massively bad consequences if they get it wrong and the rear locks up on the motorway etc or they cannot get full pressure with one pump??

[Dan]

Because once you have fitted those new cylinders, the brakes will be the same every time you use them whether they work well enough or not. There may not be a control that changes the way the brakes behave from one application to the next, road cars are not race cars. There is far more for a driver to pay attention to outside a road car than a race car and drivers aren't expected to be as skilled so the actual controls must be something you don't have to think about adjusting from one corner to another.

7 1/2" discs were most commonly fitted with a 0.7" M/C and 5/8" rears from factory. This gives a good balance. The Innocenti however used the same master and calliper with 11/16 cylinders. The difference here was the remote servo which I think was on the front circuit only so the rears needed a little more for balance. I've been using the Cooper S discs with 5/8" rears and the intergral servo master for years and find it a good mix. Of course this all refers to road use but should be a good starting point for you as you will have some balance adjustment available.

[Dusky]

Thread ressurection jeeha!
Anyway, is it advisable on all cars to swap to the smallest cylinders as possible?
Running standard 8.4 discs and servo myself, and I'm fitting all new parts, so need to decide on wich cylinders to use too :)

[Ethel]

Smaller cylinders will give you a bit less pedal travel.

 

Looking at the original question, the fluid pressure will always be the same throughout, the various piston areas will proportionally divide up the force the pistons apply. Going from .75" to .5" will give you less than half the force at the same pressure (but the servo will roughly double the available pressure). If you are retaining the same brake proportioning valve you'll still get half the force as the valve will isolate the rear at the same line pressure.

 

The .5" cylinder was fitted with a 0.7" master, if you're using the horizontal master (0.8") you'll get 7/8 of the mechanical advantage - less the servo.

 

 

Does any of this matter?

 

Switching to the smaller wheel cylinder while retaining a proportioning valve will effect your rear brake effort in a big way. I expect enough to fail a UK MoT test.

 

Stopping while reversing, or with a front circuit failure when using a FAM7821 valve, will be harder.

[Gr4h4m]

I haven't had an issue passing the mot for five years when using the .5" rwc

[MRA]

If your "discs" are using standard 8.4" type calipers then you should be using 3/4" rear cylinders

[Ethel]

I haven't had an issue passing the mot for five years when using the .5" rwc

Which brake valve are you using?

[Gr4h4m]

The mk2 rear subframe valve

[absx2]

I don`t usually like to comment on brakes as it can open up a hornets nest of rants but here goes.

I have two minis running 7.5" discs with no servo and 3/4" bore rws just because I had them in stock at the time.

The daily drive has drums all-round and the Rover Cooper has its standard set up without the PWDA piece of junk.

 All four cars have MS72 adjustable regulator valves fitted on the bulkhead under the master cylinder with the rear brake power tailored to suit my particular needs. 

The rear brakes on minis don`t need a lot of pressure being a light, short wheel base car and under heavy braking that back end gets very light indeed. Get it wrong and it will swap ends quicker than you can say "oh s**t "

In the 80`s there were a lot of minis around where the uninformed owner had decided the rear brakes were crap as they could still be moved a few degrees by reasonable force when the car was jacked up with an assistant applying the foot brake. They would then remove the internals of the pressure reducing valve thinking this was the issue creating a deathtrap.( I`ve driven a few )

 

My personal opinion is to fit an adjustable pressure regulator to the rear brakes of any mini regardless of use.

 If you have been down the side of a German 18 wheeler backwards on a narrow road because he forgot what side of the road we drive on in the Uk you would agree.

 

Hope this might help someone to avoid an accident some day.

[tiger99]

The PDWA is not a piece of junk. But you should be using the thing commonly called a PDWA, which is no such thing, because it doesn't have the warning switch. That is done by the float swith on the reservoir lid. The specific valve usually fitted limits rear pressure if the front circuit is working, but if the front circuit fails allows full pressure through to the rears to give you the best chance of staying alive. By using a fixed rear type limiter, whether fitted at front or rear, you lose that and a front failure leaves you with only the severely limited rear pressure to stop. Much more likely to be fatal.

Personally I prefer the smallest rear cylinders and no limiter, although the best solution, not legally available as of now, would be at least a partial ABS system for the rear brakes. DON'T TRY THAT AT HOME. It needs a substantial design team with the necessary competence and equipment.

[absx2]

Re. above post

Lets just call it a FAM7821. The reason  I called it a piece of junk is because they have failed so often over the 30 years that I know about resulting in cars that weave under braking for no oblivious reason because the shuttle valve is bouncing around or the rear brakes lock up too easily or both.

I even had one that would not fully allow a circuit to release. I can`t remember which as some had the hideous diagonal split that in the event of failure would send you into a backwards spin anyway.

Thats why I personalty prefer a well maintained set up without that valve. I can see the benefit of a dual circuit if it actually worked properly but as you have pointed out the float switch will give you an early warning of fluid loss. 

I feel the chances of having a tail spin situation due to heavy braking are greater than losing a brake circuit other than having a hub torn off in the hedge you just found yourself in because the car swapped ends :) I wonder how many race and rally minis have a FAM7821 fitted ??

AS for a partial ABS  thats why I use the MS72 on the rear circuit to have the rears just working making it impossible to cause a tail slide under heavy braking.

I initially set it up on an airfield driving through various real life scenarios until I was personally happy with the set up. 

DISCLAIMER

This is is to share my personal opinion on my cars only . I do not personally advise anyone to remove or modify factory fitted brake equipment .

As TIGER99 says    DON`T TRY THIS AT HOME.

Edited by absx2, 30 May 2016 - 09:19 AM.

[MRA]

Changing parts on Minis has been part of the heritage since the first on rolled off the forecourt.

 

However there are serious legal implications involved with certain parts of your car, brakes being one of them, making them better means they could pass an Engineers inspection and or examination (don't confuse this with MOT,) however using earlier systems on your car will not pass as it would be deemed to be inferior than design intent, if you were involved in a serious accident (hopefully you never are, but we know the chances are still possible) and the police called for an Engineers report you would be screwed, the chances of serving in her Majesties hotel would be high, even if you bought it like that, it is still your responsibility to ensure the safety of your car.  It's all in the construction & use manual.

 

As far as the FAM7821 valve is concerned, the valve is not at fault, poor servicing is the main issue, if you choose not to change your brake fluid at recommended intervals or better then expect to have issues with your car, it is corrosion due to moisture that stops most of these from working, not the design of the system.

 

Blaming the device is like blaming your engine when it fails, just because you didn't bother to change the oil on specified intervals of time or mileage whichever happens first !

[Ethel]

There's an argument that, if you modify other aspects of your car, you may need to modify the brakes to at least restore their original efficiency. As far front to rear proportioning goes, there are already huge parameters in varying road surfaces, gradients, passenger/luggage loads and how they are distributed.

 

I'm surprised inertia valves aren't more popular, possible as close as we can legally get to that partial ABS.

[Spider]

I'm surprised inertia valves aren't more popular, possible as close as we can legally get to that partial ABS.

 

In every non original car I build, I always ditch the POS FCPR Valve (often called a PDWA valve) or the Pressure Shut Off Valve in the earlier cars, and fit the Inertia Valve.

 

IMO, the FCPR Valves are a really dumb way of getting the desired result. They can sometimes be serviced, but not designed to be. There are way much more simpler ways of getting the job done and usually these are better ways - enter the Inertia Valve.

 

Some Minis were fitted with PDWA Valves, I think these were limited to the diagonal split systems.

[Ethel]

Early ones before it was incorporated in to master cylinder. I'm not 100% sure if any front rear splits retained it. It would have been confusing if you had a rear end pressure failure that could be shut off by hammering the pedal hard enough to close the brake limiting valve.