16 replies to this topic

[crock]

Trying to figure out why my rear brakes always lock first. I think it is an incompatibility with the hardware being used. The car is built from a Heritage shell using Cooper S front discs with EBC green pads, 1/2" rear hydraulic cylinder on the drums, and a single circuit .70 inch Cooper S master cylinder. I am unsure which brake proportioning valve is used but looks like a Mk1 style. Anybody have some idea why the rear brakes lock first? The disc brake hardware in the front all seems in order. I have the car off the road for major rebuild and now seems like the right time to swap out any parts.

[alpder]

Green pads as in "EBC GreenStuff ™"? I tried those years ago on a kit-car after believing the maker's hype. I found them very poor for road driving - because I never got them hot enough. Got waaay more braking power using regular pads from a motor factor. So if the front pads require a lot of push on the pedal to get them working, then maybe even 1/2" rear cylinders will lock the rears - though the limiter should still stop it happening.

 

Maybe the guts have been taken out of the rear limiter (or it's just got old and stopped working.) I found that you can tell if the Mk1/2-style rear brake limiter is working by trying to bleed the brakes too vigorously: open the rear bleed-screw wide and get someone to stamp on the pedal... a click and sudden cessation of flow means the valve has done its thing. It's not really intended to work that way (operating by flow-rate rather than by pressure) but the design of it means that it does.

Edited by alpder, 22 March 2025 - 08:50 PM.

[miniGTS]

I think the rear wheel cylinders should be 5/8"

[alpder]

There used to be a chart (here: http://www.minispare...tion~Chart.aspx) which detailed every production brake combination, including which rear brake limiter it used and with what rear cylinder size.

 

Unfortunately, MiniSpares' new website doesn't open at that link, and I can't figure out how to navigate to the chart on the new site. Perhaps someone here knows how to?

[Spider]

There's some other issue there.

With 1/2" rear cylinders, they give such little braking that you don't need a rear valve at all.

I'd suggest checking them, it sounds to me like you have 3/4" cylinders back there.

[crock]

I have tried 3/4", 5/8", and now 1/2" rear cylinders. The 1/2 inch cylinders work best but they still lock too early. I read the article by Kieth Calvers. The article suggests that I bin the brake proportioning valve and either just replace it with a Mini Spares 3H2424 3-way brake union and 5/8" rear cylinders, or use the Mini Spares MS72 adjustable "race" proportioning valve with 5/8" cylinders. Does anyone know if the MS72 adjustable proportioning valve is a direct replacement fir for the standard Mk1 proportioning valve or does it require some modification to fit?

[imack]

Assuming the rear brake shoes, drums and cylinders are in good condition, I'd try replacing the EBC pads with a different brand. Personally, I've never liked EBC pads whether in mountain bikes of cars, they always seem to lack any bite and fade easily.

[absx2]

I use an adjustable rear brake limiter on every mini I have and have had as it`s just so easy to fine tune the brakes regardless of the rear wheel cylinder size.

 

Please do yourself a big favour and get rid of the EBC Greenstuff pads and fit standard pads like AP GBP103MS

[nicklouse]

As mentioned there is loads of info on Calvers pages if you want to have a read.

https://www.calverst...ias-adjustment/

[Lplus]

Surely the brake proportioning valve should ensure the rears don't lock at all?  Also the cooper S was provided with a servo for a good reason - the 7.5 in discs need a lot of pressure to come close to locking the fronts.

 

That said, even if a servo is fitted to increase line pressure the rears shouldn't lock  (on a dry road) if the proportioning valve is working properly.

 

There's the option to just fit the servo to the front brake line to avoid such high pressures at the rear.....

[nicklouse]

Surely the brake proportioning valve should ensure the rears don't lock at all?  Also the cooper S was provided with a servo for a good reason - the 7.5 in discs need a lot of pressure to come close to locking the fronts.

 

That said, even if a servo is fitted to increase line pressure the rears shouldn't lock  (on a dry road) if the proportioning valve is working properly.

 

There's the option to just fit the servo to the front brake line to avoid such high pressures at the rear.....

Sorry you miss the purpose of the servo. It does not increase the line pressure. It just reduces the amount of pressure you need to apply to the pedal. 
 

the rear valve shuts off the pressure to the rear at a preset amount. (Adjustable).

 

fitting a servo to the front only is a poor option.

[Lplus]

 

Surely the brake proportioning valve should ensure the rears don't lock at all?  Also the cooper S was provided with a servo for a good reason - the 7.5 in discs need a lot of pressure to come close to locking the fronts.

 

That said, even if a servo is fitted to increase line pressure the rears shouldn't lock  (on a dry road) if the proportioning valve is working properly.

 

There's the option to just fit the servo to the front brake line to avoid such high pressures at the rear.....

Sorry you miss the purpose of the servo. It does not increase the line pressure. It just reduces the amount of pressure you need to apply to the pedal. 
 

the rear valve shuts off the pressure to the rear at a preset amount. (Adjustable).

 

fitting a servo to the front only is a poor option.

 

The servo increases the downstream line pressure for a given load on the pedal - taking it from hulk levels to human levels to get good braking force out of 7.5 " discs.  Both my minis have 7.5 in discs with aftermarket servos to the front brakes and they work just fine.  That said they are dual circuit but I don't see the problem with a servo to the fronts only on single circuit.  I suppose if the rear proportional valve is working properly the servo might just as well be in the main line from the master.

[NLinPEN]

There used to be a chart (here: http://www.minispare...tion~Chart.aspx) which detailed every production brake combination, including which rear brake limiter it used and with what rear cylinder size.

 

Unfortunately, MiniSpares' new website doesn't open at that link, and I can't figure out how to navigate to the chart on the new site. Perhaps someone here knows how to?

Do you mean this page:  https://www.minispar...ap0109/page/86/

 

 

[crock]

Thanks everybody! I will try different brake pads and the MS72 adjustable proportioning valve and see what happens when the car gets on the road.

[cal844]

I'd highly recommend mintex 1144 for a road car, I have standard EBC pads (not performance type) that come with a 7.5 disc conversion kit, they're absolutely junk, overly dusty and no feel to the pedal