Master cylinder for 4 wheel discs

[RonB]

Why is it so cryptic selecting a master cylinder for a wheel disc setup.
Hasn't this been done thousands of times with great success.

I here, use what you have, 944 Porsche, '67 and later dual circuit with 2lbs residual pressure valves,
and others.

20.6mm?
Can anyone clear this up for me?
Thanks

[Bruce2]

It depends on the calipers you are using.
If using Karmann Ghia fronts, and the commonly available rear kits, all you need is a stock replacement dual cct master cylinder. You don't need any check valves, or a bigger Bus size MC.

[Marc]

We have two Beetles with Type III front calipers and Porsche 924T/944 rears; both use the stock 113 611 015BD 19.05mm dual-circuit master cylinder, with no residual-pressure check valves.

[Theo]

I'm running aftermarket front and rear 4 lug kits on my 69 Ghia and find the bias to be way too much for to the front. I've tried a couple of master cylinders with no change. I would put the bias at 80/20. I believe stock is much closer to 60/40.

The brakes work but the fronts lock up long before the rears are even 1/2 way to lock up. I race auto cross and do OK with this setup but on the street I would consider these brakes to be dangerous in an emergency stop. Frankly I would not recommend this setup for street use at all. I have the same setup on my Baja and it is worse because of the huge rear tires. I have used residual valves (no difference) and have purchased a bias valve to tune down the fronts a notch (bias valve not installed yet).

I would venture to say the cheap aftermarket 4 wheel disk setup with the available master cylinders is poorly engineered for our cars and should be used with caution.

Perhaps using brakes from other models is a better idea?

[Bruce2]

Theo, what size are your tires?
If you reduce your front tire dia, you will get that effect.

I recall with my stock Mex Beetle (4 bolt discs/drums), when I got it, the bias was perfect and the braking was excellent with the stock 155/80/15s. Since the gearing in these cars is way too low, I put a pair of 165s on the rear. I now notice the braking isn't as good, and the bias has suffered.

My other Beetle has early single pin KG front discs, and 914/4 rear discs. The first tires I put on it were 195s and 225s. I got lucky, because the bias was excellent on it. I tested it once by hard braking on a flat empty road from about 90. While braking, I gently lifted up on the E-brake, and with minimal effort, I could get the rears to lock very slightly, indicating they were on the verge of locking up by themselves.

I see two ways to reduce your front bias. Increase the front tire dia, or bigger rear brakes.

[Jadewombat]

viewtopic.php?f=51&t=139683

I run near 50-50 bias on 924T brakes all around and dual MCs.

[Theo]

I'm running 14 inch rims on the Ghia with very short tires. I think they are 22 inches tall. Same size on all 4.

The stock Ghia brakes (front disk - rear drum) worked very well and the bias was perfect. The change to 4 wheel disks using the low buck kits upset the braking bias.

Any thought on the bias valve? I will try to get it installed soon.

T

Theo, what size are your tires?
If you reduce your front tire dia, you will get that effect.

I recall with my stock Mex Beetle (4 bolt discs/drums), when I got it, the bias was perfect and the braking was excellent with the stock 155/80/15s. Since the gearing in these cars is way too low, I put a pair of 165s on the rear. I now notice the braking isn't as good, and the bias has suffered.

My other Beetle has early single pin KG front discs, and 914/4 rear discs. The first tires I put on it were 195s and 225s. I got lucky, because the bias was excellent on it. I tested it once by hard braking on a flat empty road from about 90. While braking, I gently lifted up on the E-brake, and with minimal effort, I could get the rears to lock very slightly, indicating they were on the verge of locking up by themselves.

I see two ways to reduce your front bias. Increase the front tire dia, or bigger rear brakes.

[Piledriver]

I've just converted the rear on my square to 914 discs and 38mm piston Audi (VW mk4-like) floating calipers..
Even with the self adjusting handbrake working properly, I have (IMHO) too much pedal travel.

The MC on my T3 is an ATE with a "16" cast into it that usually indicates bore size, but there's no telling these days short of taking it apaht....

Is the std 19mm Beetle MC a reasonable swap? other suggestions?
(I've already dual circuited the front BMW 4 pots, the the front circuit currently only feeds 2 32mm pistons in the upper piston on the front calipers, so bending/replacing a little brake tubing is no issue)

Some of the allegedly "compatible" Centric units on ebay list being 13mm bores, assuming they even read what they are posting.

[Marc]

To the best of my knowledge all dual-circuit Type III M/Cs are 19.05mm; there was a 20.46mm one used on early Squarebacks but I think it's single-circuit. Never tried grafting Type I into Type III or vise-versa; comparing them side-by-side should tell you if they'll physically fit - from there, plumbing differences would be trivial for you.

http://www.thesamba.com/vw/forum/viewtopic.php?t=194303

[Piledriver]

Thanks for the info, Marc, the ATE master I have looks like the std port layout (switches on driver side) but definately has a "16" cast in, (or at least I swear it looked like it, looked 3X to make sure) which seemed odd.

After a proper bleeding with the help of my son, they feel OK, and will probably get better as the rear discs/pads get fully acquainted, may need to adjust the parking brake cables a tiny bit, have not done that.

May look into splitting the rears as VW did on the Swedish models (gotta love Swedish models) at a later time along with the Mercedes 22mm MC, but I'm going to stick a fork in it for now.

The Bettle master appears like it would work by drilling out the threads on the mounting flange and adding a T for the front lines.

Grr, forgot to take the pictures, back out to do that.

[Piledriver]

Pics as promised.
Brakes still feel spongy after a good bleeding, suspect it's all the additional rubber in the system and/or the flex in the floaters out back, felt fine with the drums and dual front hoses, just very sensitive to any play in the rear adjustment.
Master is a 16mm ? by the # on the side, probably a contributing factor.
May just pick up a reman 19mm for $40, just about doubles my cost to do it so far
(posting here as the ~same or very similar setup could be done on a T1)

I cleaned up the welds, and added some tie downs for the parking brake cable and brake hose after this shot, keeps`emfromfloppin.

front-dualcircuit.jpg

T3-rearaudicalipers.jpg

T3-16mm-mastercylinder.jpg

[Piledriver]

After cutting an old steel brake line and screwing that into the bleeder port and sucking on THAT (good seal) with the mityvac. The high point in those top mounted brake lines trapped a lot of air, they work great now.