bigger master cylinder?

[trevorbrady]

I fitted a rear disk brake conversion to my bug this weekend, with Porsche 914 disks and Peugeot 504 calipers (no, I don't know why the manufacturer chose this caliper either )
I had everything bolted on, plumbed in, adjusted and bled only to find that the stock VW master cylinder can't move enough fluid to have much of an effect on the rear pistons. The result is very soft spongy brakes, and the pedal hitting the floor with very little effort.

Is there any other master cylinder that will bolt in place of the beetle one but have a larger capacity?
I'm thinking of ditching the 504 calipers and sourcing some Golf ones or something. I've already discovered getting parts for the 504 calipers is a bloody nightmare...

BTW, I ended up switching back to the rear drums, cos I had to drive the bug and just don't like the idea of having no brakes!!

[Alex]

Have a look at the Kerscher MC.
I think it is 21/19 in size and is used for the Kerscher disk brake kits.

Alex

[Steve C]

Hi

I used to have a similar setup, 914 rotors with Golf calipers. They had 36 mm single pistons and worked very well with the stock mc. What size pistons do your Peugot calipers have? Sometimes when you bleed your brakes the MC seals can get damaged when they run over a rough spot in MC that they normally dont travel over except when bleeding the brakes, thatmight explain your lack of pedal now.

Cant stop thinking about that Cheech Marin movie "Born in East LA" every time I hear the word Peugot.

Steve C

[trevorbrady]

I can't say what size pistons are in the Peugeot calipers, but looking at the casing and the end of the piston that pushes the pad, it looks like something big, in the region of at least 40mm.

The master cylinder is only about 12 months old. I would hope there wouldn't be too much corrosion on it!

[MdR]

I have a 944 split bore (Turbo?) mastercylinder fitted to my chassis. It's nice & shiney, made of Aluminium with 23/20 bore sizes. I can't tell you what it's like to use 'cos the car's STILL in bits BUT it fits the standard mount point perfectly, with a shortened pushrod. The m/cyl has no provision for a std bug brake light pressure switch. I have used a T-piece in the line & fitted the switch to that. A friend has fitted an Audi 100(?) m/cyl & that went in with no problems. His car is in bits too.

[Steve C]

Hi

This part of an email that I got from Gropher, a poster on STF, I havn't heard from him for a while but he said some interesting things in relation to M/C sizing

Steve C

====================================

I've misplaced my cheat sheet on which I had evey Porsche caliper's and rotor's specs: piston sizes, caliper dimensions and mounting style, pad area, rotor dimensions & hat/hub offset,
swept area, etc... And of course part numbers and new prices. If I find it, I'll email it to you. I remember most of it off the top of my head, as brakes were a speciality of mine for a number of years.

I wouldn't fret too much about the master cylinder size. I've driven in more cars with poor brake feel because people alculated the exact M/C size by following their original car's ratio of M/C to piston. Very scientific, yes, but that level of accuracy can be overkill. Sizing the M/C can be as much to your driving style -- some people like more pedal travel so that the brake pedal ends up lower when they start to bite, leaving the pedal
about level with the accelerator for easier heel-and-toe operation.

I would say from my experience that you shouldn't go larger than a 19mm M/C. When I've installed 4-piston Turbo or monobloc kits on the front of 914s but retained the dual piston calipers in the rear (after much bias valve adjustment), the 19mm M/C gave a high, firm pedal with adequate range to control modulation. The same is true even when 4-piston rear Turbo calipers are installed. The 914's stock 17mm M/C will even work with the 4-piston calipers up front, but you have almost as much pedal travel as stock. The advantage is little pedal effort is required. A 23mm Mercedes M/C unit is terrible -- little travel, very hard (lots of pedal effort required) and no possibility of modulation

-CC

[ricola]

Hi,
Do you know the specs for the Audi m/c?
I found that a stock bug m/c was not sufficient for the early 944 floating caliper brakes, it didn't push enough fluid to the front. As a half way stage I did have 944 on the rear and bug discs on the front and this gave a very nice bias and feel. As people are saying that the stock m/c is fine with the turbo brakes I can only guess that the early caliper pistons withdraw further from the disc when you lift off the pedal resulting in a larger swept volume.
I am currently using a 944 turbo m/c but find this gives too much bias to the front circuit. Are there any other in between options?

Rich

[MdR]

Rich
Hi
Ask Moog (Nigel) NCHowe*NOSPAM*@ricardo.com (remove the obvious!). He's got Audi Coupé front calipers on T4 discs & Golf rears on 914/4 discs. I know he needs to know about radiator positions when converting to watercooled power so I'm sure you can trade info!
Martin

[Steve C]

Hi

Im using 996 rears on the front of bug with a standard 19 mm MC, the pedal is fine. My advice is to just try a stock MC and if your sure you have a problem, move on from there.


Pistons in my front Brembos, 37.5 mm & 26 mm

37.5 / 2 = 18.75 x 18.75 = 351.5625 x 3.1416 = 1104.46875

26 / 2 = 13 x 13 = 169 x 3.1416 = 530.9304
so total area is 3270.7983


Stock Beetle, calipers 40 mm

40 / 2 = 20 x 20 = 400 x 3.1416 = 1256.64
total area is 2513.28

Steve C

[kdanie]

Trevor, what MC are you using now? Stock VW, single circuit or dual? Maybe a N/A 944 MC would work? I know it bolts in ok but I haven't had a chance to drive mine until the engine is installed and plumbed. If you pick a MC that is too large a dia. you can lengthen your pedal an inch and increase your leverage.
ken

[ricola]

Does anybody know the bore diameters of the NA 944 m/c?

[trevorbrady]

Originally posted by kdanie
Trevor, what MC are you using now? Stock VW, single circuit or dual?

I'm using the stock VW dual circuit MC.
Good point about the 944 one, do you know what bore it is?
Ideally I am looking for something that will bolt in place of the old one. If this is a bigger bore, it should be ideal...

[kdanie]

I opened mine up to make sure it was clean and worked ok but never measured it.
ken

[MdR]

Originally posted by kdanie
I opened mine up to make sure it was clean and worked ok but never measured it.
ken

Ken
My m/cyl has the bore sizes cast into the outside (top) of the body. Maybe yours has too.
Martin

[MattKab]

'My '79 has a stock 1303 m/c and it operates 4 Brembo 4pots perfectly. You can increase the length of the pushrod action(???) by moving the pedal stop forward, this brings the pedal back and down.

Due to the amount of writings about other, larger bore m/c's being recommended, I have had my brakes tested on the rollers at the MOT station. The inspector was impressed.

When I'm bleeding the air out, I get a feel of how much fluid is 'behind' the pedal, front and rear. I'm perfectly satisfied.

I'm not the first to use a stock m/c with this conversion.

My car was converted to RHD and the pedal stop is maybe a couple of mm forward and needed no adjustment before, or after the brake conversion.

Heavy braking from high speeds, even on bends in the wet gives me no concerns.

Possibly the precision of the Brembo calipers keeps the brakepad retreat to a minimum, less 'backlash' for want of a better word, there is considerably more area of slave cylinder than the 4drum Bug.

I know moving the pedal back, as stated above does nothing to alter the amount of fluid displacement, it does alter the mechanical advantage of the pedal/lever.

'Something to think about, consider all the relavent lengths and angles of the stock Bug brake pedal geometry, would it be the case that maximum mechanical advantage of the pedal coincides with the point where the energy is transfered during braking?
As we all used to know, the drums need constant adjustment, as wear gives loss of material, brake pedal 'bite' goes down to the floor. When the brakeshoes are nicely bedded-in and adjusted to tolerance, would it be that the brake pedal lever passes through the point of maximum mechanical advantage, as normal wear occurs before re-adjustment is needed?

Is your rear brake conversion part of a kit? Is it supposed to be compatible with the stock m/c?

911 between '65 and '85 had a m/c bore of 19.05mm if that's any help.

MattKab