13 replies to this topic

[theoldmini]

Just acquired an older mini 1975 and the braking system does not have a servo.  The system is dual circuit was wondering which brake cylinders to fit to the rear as going to overhaul all the brakes.  I understand there are different bores to the cylinders.  The brakes are very poor and the fronts are 7.5 inch discs.  I will also change all the fluid at the same time.  All help appreciated.  Thanks

[absx2]

A 1975 mini with dual circuit would use a 3/4" (19 mm) rear wheel cylinder.

 

The 7.5" S brakes on a Cooper S would have used 5/8" rear wheel cylinders with a 21A1774 regulator valve on the rear subframe and of course a single circuit master cylinder and a servo.

 

You will have to stick with the 3/4" bore cylinders unless you start from scratch with the braking system as the pressure is regulated for that bore size, if you fitted 5/8" rwc`s the rear brakes would be worse.

 

I have always found S brakes lacked any feel without a servo being quite hard and dead at low speeds but great a higher speeds unlike the drums that are great around town with plenty of bite and feel but useless for spirited driving but loads of people run with un servoed s brakes and find them just fine.

 

Edit.

I have a P reg Clubman 1976 with a single circuit brake system so if it was mine I would fit all the correct parts for Cooper S spec as in the GMC172 master cylinder etc.

You should have a regulator valve on the bulkhead with dual circuit brakes ( FAM7821 ) Fitted from 1978 on according to mini spares.

These things can play up a lot so by going to s spec you can remove it and use the 21a1774 or an ms72, them fit 5/8" rwc`s and a servo if you wish.

 

Modifying brakes from standard is always a sticky subject from an insurance point of view so make sure you tell them and being a 1975 car i can`t see single circuit being an issue.

Edited by absx2, 17 December 2017 - 09:39 AM.

[Spider]

I'd suggest 5/8" Bore rear Wheel Cylinders, however, if the brakes are poor then I wouldn't be looking to anything on the rears to improve them.

 

At a guess, I'd say the pads you have are too hard and / or the discs are glazed.

 

If they have no booster, they will need a fairly heavy foot to get them to work. don't be frightened to stand on them.

[whistler]

I have a 72 clubman I'm just replacing the rear wheel cylinders on. They are as absx2 mentioned but I have 8.4 discs on the front, no servo. Brakes were good even before I discovered the fluid leak on rear RH side. As Moke Spider said make sure you don't have hard competition pads fitted or that they are not glazed.

Edited by whistler, 17 December 2017 - 10:10 AM.

[theoldmini]

So to clarify 19mm rear cylinders change pads to soft ones and leave the system as dual circuit with no servo. What difference would the 5/8 cylinders make  Thanks for all input.

[whistler]

So to clarify 19mm rear cylinders change pads to soft ones and leave the system as dual circuit with no servo. What difference would the 5/8 cylinders make  Thanks for all input.

Not necessarily 'soft' pads, just standard ones.

[Spider]

So to clarify 19mm rear cylinders change pads to soft ones and leave the system as dual circuit with no servo. What difference would the 5/8 cylinders make  Thanks for all input.

 

Having too big a wheel cylinder size on the back wheels makes them more prone to locking quite easily and you'll find that when it does this, the car will swap ends very fast. I found 3/4" Cylinders too big for this reason and going to 5/8", which were the original size for a Cooper S (that was fitted with the 7.5" Discs) cured that issue.

[carbon]

+1 for keeping the rear wheel cylinder size as small as possible, for reason of rear wheels locking under heavy braking in wet conditions. Using 3/4 inch bore gives 44% more force than 5/8 bore for same line pressure.

 

I have 5/8 inch on the rear with 7.5 inch discs up front, no servo. Single line, so the pressure regulator is on the rear subframe. This has been set to give minimum braking on the rears. Just enough on the rears to pass MoT brake test, no more.

[Sprocket]

+1 for keeping the rear wheel cylinder size as small as possible, for reason of rear wheels locking under heavy braking in wet conditions. Using 3/4 inch bore gives 44% more force than 5/8 bore for same line pressure.

 

I have 5/8 inch on the rear with 7.5 inch discs up front, no servo. Single line, so the pressure regulator is on the rear subframe. This has been set to give minimum braking on the rears. Just enough on the rears to pass MoT brake test, no more.

 

Just enough? ...... you can go with less than the 5/8" on the rears and still pass the MOT requirement.

 

 

I fitted the 1/2" cylinders on the rear of mine with the standard single line brake bias valve mounted on the rear subframe. All the braking is on the front on my car, with big 10.25" disc four pots up front and wet grip A tyres. I too didn't want the car swapping ends unnecessarily, but was worried about the MOT thinking I might have go up to the 9/16" cylinders. Braking effort on the rear is almost nothing, you can turn the rear wheels with the brakes applied.

 

On the brake rollers, the fronts almost maxed out the scale, the rears barely left the bottom of the scale lol. Overall efficiency was way above the required 50% so it depends how good the fronts are as to whether less on the rear is a pass or not. The fact the hand brake still achieves greater than 25% shows the rear service brakes are 'limited' rather than non functional.

[nicklouse]

One thing to remember the locating pin in the housing is positioned to match the back plate for the brake set up so you may need to re drill the back plate.

 

or they were when new from BL/rover

[Sprocket]

One thing to remember the locating pin in the housing is positioned to match the back plate for the brake set up so you may need to re drill the back plate.
 
or they were when new from BL/rover

Yes, thats right, the pins are still on the other side of the cylinders. Simple 5min job though to re drill the back plates on the car when the cylinders are off.

[theoldmini]

So the consensus of opinion is 5/8 rear cylinders non servo and 7.5 discs at front with standard pads.  I will get on with that.  Thank you

Edited by theoldmini, 19 December 2017 - 09:45 PM.

[absx2]

just be aware that if you have the bulkhead regulator valve that is normally fitted to dual circuit brakes the 5/8" cylinders may give too little braking effort to the rears as it was designed to run with 3/4" cylinders.

The models that originally used the 5/8" cylinders used a 21A1774 valve.

It may well be just fine but it`s worth a mention and you would only find out at MOT time with an unforgiving tester.

 

I use a MS72 and adjust the rear brakes accordingly, that way you can mix and match parts to work together.

 

The 5/8" cylinders may be OK with your set up and as already mentioned too much rear braking is bad news. I can turn my rear wheels by hand ( big hands ) with the brakes full on and it passes an MOT. Its better that way than swapping ends in an emergency situation. 

[Spider]

 if you have the bulkhead regulator valve that is normally fitted to dual circuit brakes

 

Does anyone have any data from AP on these?

 

Having stripped a few down, they are a pressure shut off valve, that in very simple terms, has a similar effect to the old shut off valve that was fitted in the single circuit brakes, however, I don't know at what pressure these shut off and / or if there were different pressures.