Master cylinder is from 68-69 VW Bus with manual brakes - Available new from Bughaus for $140 or rebuilt A-1 Cardonne #11-1558 from local parts store for about $30. 7mm Brake hose, 10mm bleeder(to close off extra rear outlet) and nylon master cylinder connector pipes (small) all came from Bughaus. Lines from reservoirs to master cylinder formed from 40 inch pieces of 1/4" steel brake line. Plunger from brake pedal needs to be turned down slightly or master cylinder piston drilled out slightly for proper mating. Pretty simple and straight forward.
Ted, I stumbled upon this same dual master cylinder several years ago and made the same conversion as you. It works perfectly. I took a couple of brakeless thrill rides before the conversion; once when old master cyl failed without warning, and once when a steel brake line blew out under floor. I paid $28 at Napa autoparts for the dual. I think the original single master cylinder is just trouble waiting to happen.
So I bought the aforementioned, and successfully used dual circuit Cardone 11-1558 MasterCylinder from amazon last year, and kept it safe till this past week when i was ready to put it in.
Then i noticed something: The original master cylinder (reportedly the same one VW used on early/50's split-window buses)..has tapped holes that the brake fluid travels thru.
But the dual circuit unit that folks here have used (reportedly used on 68&69 vw buses), has these mysterious valve-like devices that the lines screw into, rather then just screwing directly into the cast Master cylinder.
I had trouble not being able to purge air out of the front chamber that feeds the fonts (or maybe getting air leaking back in), so i tried to bench bleed it....but was still getting a sucking sound, and air in the front chamber. So i emptied it out and said "what the hell are these gadgets anyway, are they causing the problem? They arent on the original master". So i removed them out of curiosity and tried blowing thru them , and to the mils capabilities of human mouth-pressure, are much more resistant to airflow than the original master that blows as smooth as a straw!
What the hell are these things?....and could they be causing air to be sucked back in? (ps - i ordered another master from a place that sell new ones, in case the seals on the Cardone reman one are bad(
But what are these restrictive devices that are not on the original masters?
Aha! - found the answer! - They are "residual pressure valves": small check-valves, designed to keep a little bit of pressure in drum brake lines to reduce excell pedal "travel" or the need to "pump up" the brakes if they pull a but too far away after sitting. They were a feature on the 68/69 era VW master cylinders for VW's that had 4 wheel drum brakes. They eventually eliminated them, when brake servos became more popular. Amphicars are not too picky, and have effective brake-shoe adjustment cams to keep the shoes just barely away from the drum. The RPV's can be left as-is... but might make air-bleeding tougher, so here is a thread where they are gutted and used simply as pass-thru adapters (because that model 22mm dual master cyl is drilled for M12 to accept the valves...but Amphi brake line fittings are M10 - so you DO need the hex shaped metal nut to screw into!
"Residual pressure valves are normally used in drumbrake systems.
These valves maintain a slight static pressure in the hydraulic system at all times, even when the brake pedal is released.
This residual pressure holds the wheel cylinder piston-cup lips tightly against the cylinder walls and prevents air (and water) from entering the system when brakes are at rest."
I used the 1968-69 Bus master cylinder in my Amphicar and it works great, I had no problems with the residual pressure valves. I replaced the metal lines with new bronze ones and switched to silicone brake fluid. I did run into difficulty creating the double flare on the brake lines and wound up purchasing a small tool from Eastwood which worked very well. I ran two hoses up to a new two section reservoir, a generic one from a local aftermarket VW dealer. I believe the original VW bus configuration has a reservoir that sits right on top of the master cylinder, with a single hose or pipe running from that one to another reservoir (that has a filling cap on it) above it. Glad to have the twin circuits after having my brake pedal go to the floor a few summers ago.
The 1971 VW Beetle master cylinder has the brake line ports facing the wrong direction; they are clocked wrong for the Amphicar - it'd be difficult or impossible to attach the lines to the master cylinder due to clearance issues with the fender well.
I already had a brake pedal rod that was reduced in diameter so no issues with it.
I kind of got off on a tangent with the residual pressure valve topic. Now that i understand what they are, lem me ask everyone a question regarding their dual-circuit master cylinder:
I had the following problem - maybe you can help me:
I bled the rears, and got firm pedal (as you probably know, both rears are "tee'd" from a metal line fed by the first chamber of the dual master)
...then i went to the front passenger.
when i bled it, rather than the pedal getting even higher and firmer, i lost ALL pedal firmness.... it went right to the floor.
Why would bleeding the front, cause even the isolated rear circuit (that HAD pressure) to lose ITS pressure??
I cant figure it out, but i bought another master cylinder in case somethings fuc%ed up with this one. Maybe its something else, but im stumped and thought i would try another brand of the same '68 style (recommended by a vw specialty shop)
Hi Jon, no, I didn't bleed or prime the master cylinder first. I had a relative do the footwork on the brake pedal while I bled the wheels, and he pumped it up several times initially; perhaps that's why he did it. (He's a professional mechanic with 40 years experience.) I'll ask him about the valves.
Our local air cooled VW shop told me that they prefer the German made master cylinders due to higher quality than others - at a much higher price. I think mine was from Brazil; I bought it from CarQuest a few years ago. I did clean and paint the exterior before installing it in the car, to prevent rusting (cosmetic reasons).