Split brake master cylinder

[360gts]

Want to upgrade the bake system on my vintage race car....currently using the standard large can S master....
Which one works best on a front/rear split system? Somefrord have this one...anyone using this on their MK1 disc setup?
http://www.somerfordmini.co.uk/eshop/in ... ts_id=3509

[iain1967s]

If you're looking for period parts, BMC Special Tuning for Cooper S and 1275GT both used the GMC159 split circuit master cylinder, with the vacuum servo/booster in the (lower piston) front brake circuit, and the (upper piston) rear circuit left un-boosted. Part number for the kit was C-AJJ3388, there are some details on Mark's ST document archive and this previous thread.

My '67 S has exactly that setup, I think from its days as an ice racing car in Minnesota. It works well, but it's a pain in the ass to bleed - I ended up using 4 one-way ezbleed valves, one per wheel, to be able to do it properly. You also need the corner-cut version of the airbox (if using the standard pressed steel item for twin SU's) due to the extra height of the GMC159 reservoir.

They do come up on eBay from time to time, but crazy expensive... for example https://www.ebay.com/itm/323808414348 but even then you still need to find or make a matching rod to fit the mini pedal box.

[croc7]

If you're looking for period parts, BMC Special Tuning for Cooper S and 1275GT both used the GMC159 split circuit master cylinder, with the vacuum servo/booster in the (lower piston) front brake circuit, and the (upper piston) rear circuit left un-boosted. Part number for the kit was C-AJJ3388, there are some details on Mark's ST document archive and this previous thread.

My '67 S has exactly that setup, I think from its days as an ice racing car in Minnesota. It works well, but it's a pain in the ass to bleed - I ended up using 4 one-way ezbleed valves, one per wheel, to be able to do it properly. You also need the corner-cut version of the airbox (if using the standard pressed steel item for twin SU's) due to the extra height of the GMC159 reservoir.

They do come up on eBay from time to time, but crazy expensive... for example https://www.ebay.com/itm/323808414348 but even then you still need to find or make a matching rod to fit the mini pedal box.

I have the GMC159 installed as well and read the 'caution' note regarding bleeding front and rear at the same time. Bled mine the normal way, starting at the furthest from the MC, etc. Worked fine. The car stops when I press the middle pedal.

[Exminiman]

Might not be direction you want to go in and a bit pricey, but these from KAD seem to be very popular or you can piece together your own kit by buying cylinders etc..
https://kentautodevelopments.com/collec ... linder-kit

[360gts]

I have a few of these MC's left from the early 70's....think they are off the first MK3 minis that came into Canada with the diagonal system....
Am I right to assume that I could rebuild these with the 'front/back' kits!

[iain1967s]

If it’s diagonal, most likely GMC160.

IIRC, main difference is that the front/rear GMC159 has different reservoir split [more for front, less for rear] whereas the diagonal split is 50/50. Piston and bore sizes might also be different - I don’t know for sure.

50:50 should work fine for front/rear, might need smaller rear slave cylinders if there regally is a piston size difference, but when running a booster on the front circuit the rears get very little pressure anyway so maybe no bad thing.

[360gts]

If it’s diagonal, most likely GMC160.

IIRC, main difference is that the front/rear GMC159 has different reservoir split [more for front, less for rear] whereas the diagonal split is 50/50. Piston and bore sizes might also be different - I don’t know for sure.

50:50 should work fine for front/rear, might need smaller rear slave cylinders if there regally is a piston size difference, but when running a booster on the front circuit the rears get very little pressure anyway so maybe no bad thing.

Iain
Pretty sure these are the 160 MC's...if the only difference is in the reservoir split...don't see why these wouldn't work ....the car has an adjustable pressure valve to change the rear pressure.

[iain1967s]

I have the GMC159 installed as well and read the 'caution' note regarding bleeding front and rear at the same time. Bled mine the normal way, starting at the furthest from the MC, etc. Worked fine. The car stops when I press the middle pedal.

I think it depends which bleeding method you are using, manual up/down pedal versus vacuum pull.

My experience with the up/down method was that with a front caliper bleed open, the rear circuit would remain pressurized by the second piston, so the pedal couldn’t be pushed all the way to the floor while bleeding the front. Vice-versa with a rear cylinder bleed open and the front pressurized. That, after all, is the whole point of having split brakes - some pedal pressure remains even with one circuit open.

To avoid the problem, I bought 4 ‘motorcycle size’ one way ezbleed valves off eBay, put one on each wheel bleed nipple, opened them all and then pumped slowly up/down until all 4 corners had purged. Worked fine for me that way.

If you were using a vacuum pump to pull fluid from one wheel at a time to bleed, probably there would be no problem doing them individually as you aren’t reliant on brake pedal movement to push the air out?

[green1071]

Questions:

If you a running a vacuum servo with 7.5 disc brakes on front and drums on back. A standard MK1 Cooper S set up and you split the system with a GMC 159 or GMC 160 would you still use stock rear brake pressure regulator, adjustable regulator, or none for the rear drums?

I having a difficult time finding 'ezbleed valves' front disc calipers are different size than rear piston bleeders. Or are you just using the generic in line hose type?

thanks

[Spider]

Questions:

If you a running a vacuum servo with 7.5 disc brakes on front and drums on back. A standard MK1 Cooper S set up and you split the system with a GMC 159 or GMC 160 would you still use stock rear brake pressure regulator, adjustable regulator, or none for the rear drums?

Splitting Front / Rear you'd use 5/8" Rear Wheel Cylinders and the stock Pressure Valve.

Diagonal Splits, you'd use 1/2" Rear Wheel Cylinders and no Pressure Valve.

[croc7]

I have the GMC159 installed as well and read the 'caution' note regarding bleeding front and rear at the same time. Bled mine the normal way, starting at the furthest from the MC, etc. Worked fine. The car stops when I press the middle pedal.

I think it depends which bleeding method you are using, manual up/down pedal versus vacuum pull.

My experience with the up/down method was that with a front caliper bleed open, the rear circuit would remain pressurized by the second piston, so the pedal couldn’t be pushed all the way to the floor while bleeding the front. Vice-versa with a rear cylinder bleed open and the front pressurized. That, after all, is the whole point of having split brakes - some pedal pressure remains even with one circuit open.

To avoid the problem, I bought 4 ‘motorcycle size’ one way ezbleed valves off eBay, put one on each wheel bleed nipple, opened them all and then pumped slowly up/down until all 4 corners had purged. Worked fine for me that way.

If you were using a vacuum pump to pull fluid from one wheel at a time to bleed, probably there would be no problem doing them individually as you aren’t reliant on brake pedal movement to push the air out?

Now that I think on it, I did use a vacuum pump. Quoting the note from Seven, "one needs to bleed BOTH front and rear cylinders AT THE SAME TIME. These bold instructions are on the original sheet, and read: "Three operators and two sets of bleeder equipment will be required." I don' t know if that means to bleed front and rear on the same side at the same time or to bleed both fronts at the same time, then bleed both rear brakes at the same time. My best guess, from reading Iain's explanation above, would be to bleed fronts at the same time, followed by rears at the same time.

[Spider]

The need to bleed front and rear (or both circuits) at the same time has nothing to do with the Master Cylinder.

That instruction is only so that the Pressure Failure Switch doesn't trip when bleeding, if bleeding one circuit at a time.

You can bleed one circuit at a time, it the PFS is fitted, at the end of the operation, remove the Switch, see look inside to see which way the Switch has Tripped to, then carefully move it back to the centered position. A small Screwdriver used carefully can be used to do that.

When bleeding one circuit at a time, ideally, you'd start from which eve circuit is connected to the bottom port of the Master Cylinder first.

I gotta say guys, I'm not a fan of Vacuum Bleeder. The Brake Hydraulic system, and in particular, the seals are deigned to work with neutral to positive pressures only. Applying a negative pressure (vacuum) in the system can draw in air through the seals.

[bwaminispeed]

I use a special made up bolt to keep the PFS valve centered, and, bleed as usual......

Remove bolt and replace switch when done......

[360gts]

My original question was about the Tandem master cylinders....these were fitted to MK 3 Canadian cars from 1970 on....this was a front to rear system....they switched to a diagonal system around 79..ish.
Both of these systems used a 4 port valve that was mounted on the firewall where the 3 way fitting would have been on the previous models.
Attaching a picture for reference.
This item has a switch to warn when one of the lines failed.......looks like the rod inside would slide over to close off the failing line and activate the switch at the same time.
Am I correct in my thinking on this?

[bwaminispeed]

Yes, that is exactly right, commonly referred to as a shuttle valve......

[iain1967s]

Yes, that shuttle valve is standard fitment when dual circuit master is factory fitted on the later, and earlier export model, cars.

But the BMC/ST kit that I was describing - for converting Mk1/2 from single circuit, to dual circuit using GMC159 - doesn’t use a shuttle valve.

The installation instructions are to break the single circuit system at the front bulkhead T, connect the lower master cylinder outlet direct to the front circuit (via servo/booster if fitted), and connect the upper master cylinder outlet to the rear circuit rear via the front T (retaining the rear subframe brake limiter).

Regarding bleeding, the instruction is poorly worded, but I think the intention to bleed both circuits at the same time (i.e. one front wheel and one rear wheel), simply because both circuits need to be open in order to get full brake pedal travel. Without doing that, unless the master cylinder was pre-primed, it’s difficult to expel all the air when using the conventional up/down pedal method.

[iain1967s]

Here’s a photo of the ST single to dual conversion kit fitted in-period by a previous owner of my LHD Mk1 S.
F = front brake circuit, R = rear brake circuit, C = clutch.
(I’ll be tidying this up with some clips, next time the engine is out)

0594B68A-39BD-4A49-AAA1-4C3460CDF828.jpeg

[Spider]

This item has a switch to warn when one of the lines failed.......looks like the rod inside would slide over to close off the failing line and activate the switch at the same time.
Am I correct in my thinking on this?

There is only 2 seals inside the unit, in the form of O Rings, and these are around the more central part of the Piston and the seal the Fluid from the Switch. Operation in a Fail Mode of the Valve might restrict flow in the Failed Circuit, but it won't shut it off all together.

[bwaminispeed]

I don't think it was designed to restrict, or, stop flow, I think it's sole purpose was just to activate the warning light........

All the warning light did, was verify what you already knew, as, the pedal was already nearly on the floor, and, you weren't stopping much.........

So, good idea Leyland.......

There were two versions of the shuttle, the one as shown with O rings, and, another one that had tiny weeny cup seals......

[Spider]

I use a special made up bolt to keep the PFS valve centered, and, bleed as usual......

Remove bolt and replace switch when done......

I did mean to say the other day when you posted this - top idea. How simple !