TheSamba.com :: View topic - Dual Circuit Master Cylinder: Why NOT a 67?

Wed Mar 24, 2010 9:56 am

sventinker wrote: okay one more question. where are the check valves on my stock single circuit brake system?

On single circuit master cylinders, the check valve is internal. That's why old speed sells a modified single circuit master cylinder for disc brake applications.

http://www.thesamba.com/vw/classifieds/detail.php?id=789467

Taken from this thread
http://www.thesamba.com/vw/forum/viewtopic.php?t=169908

oldspeed@flash.net wrote: Typically the pre 66 busses we do get a modified stock master cylinder. The modification is the removal of the residual pressure check valve. For 1967 the residual valves can be disabled as well. For those who want the safety of dual circuit for a pre 67 bus we install a late model water cooled master cylinder as it has no residuals and is small bore so as not to have a pedal with no brake travel. In this application a 1967 master is also desireable but finding a reservoir bottle can be difficult. We have installed early Rabbit bottles to the 67 master and I do have the part number available and maybe even a couple of bottles.

BarryL wrote: Here's my opinion on this here discussion.

I've never seen a "check valve" on any pre '67 master cylinder or in-line. "Check valve", to me, is an unrelenting valve that would keep the brakes locked up. "Residual pressure" means that most of the fluid returns to the master after you let up but with a dual circuit system a tad stays against the spring tension so the front and rear circuits don't do a ba-dump or out of sync two-step. If your wheel cylinders need pressure against them to not suck air then something is haywire. The spring pressure pushes the fluid rather than the piston sucking in.

Look at a disc brake and you'll see how the bore fills with fluid and pushes the piston into the pad into the disc. When you let off there is no need for a spring as the pad only needs to relax against the 90 degree surface its against. Whereas a drum the lining pad needs to withdraw with a spring to keep from the entire circumference's surface dragging.

I like those numbers of 10 psi and 75 psi but I've never measured them.

Your single circuit system should have come just basic with no residual valves anywhere.

You've never seen the check valve because it's internal. Internal check valve are not unusual.

http://ecihotrodbrakes.com/brake_facts.html

Wed Mar 24, 2010 10:31 am

It keeps getting called a check valve and even the linked site calls it a Residual Pressure Valve. There is a huge difference. I've seen the insides of lots of master cylinders. I even took one to bed with me. I guess I need to see what a residual pressure valve looks like in, say, a '66 VW bus master.

This is from the link:
"Typical wheel cylinder seals only seal when there is pressure behind them. Rapid release of the brake pedal creates a vacuum in the system which causes the seals to relax and air is ingested into the wheel cylinders. Maintaining 10 PSI in the system at all times prevents this."

I guess I'll have to learn more about this so I don't appear to be a doofus. Most single circuit non power assist masters for drum brakes I've encountered have a relief valve and the piston slides back to its relaxed position which places the supply hole below the reservoir so the fluid just returns. The brake springs do all the work of pushing in the slaves which places pressure against the seals.

Wed Mar 24, 2010 11:32 am

BarryL wrote: It keeps getting called a check valve and even the linked site calls it a Residual Pressure Valve. There is a huge difference. I've seen the insides of lots of master cylinders. I even took one to bed with me. I guess I need to see what a residual pressure valve looks like in, say, a '66 VW bus master.

This is from the link:
"Typical wheel cylinder seals only seal when there is pressure behind them. Rapid release of the brake pedal creates a vacuum in the system which causes the seals to relax and air is ingested into the wheel cylinders. Maintaining 10 PSI in the system at all times prevents this."

I guess I'll have to learn more about this so I don't appear to be a doofus. Most single circuit non power assist masters for drum brakes I've encountered have a relief valve and the piston slides back to its relaxed position which places the supply hole below the reservoir so the fluid just returns. The brake springs do all the work of pushing in the slaves which places pressure against the seals.

How the residual pressure valve works is that once the system gets down to desginated psi, it acts as a check valve.

As for learning more about it, call up old speed and ask them about it. They've been doing it for years and even sell the modified master cylinders, so I'm sure they'd be able to answer your questions.

Wed Mar 24, 2010 3:00 pm

Ok. Sheesh. I took apart a couple different years and read the Bentley. Boy am I the idjit. That little thing is the check valve, huh? The Bentley even calls it the check valve. I always called it a relief valve as it let the fluid come back into the reservoir. So how much residual pressure is left in a single circuit VW bus system? On the '67 dual circuit masters the check valve is not in the bore whereas in the single circuit the check valve is in the bore.

Can I still come into the deep end once in a while? I promise I won't splat chlorine water in your eyes or do cannon balls. Please?

Wed Mar 24, 2010 4:02 pm

BarryL wrote: Ok. Sheesh. I took apart a couple different years and read the Bentley. Boy am I the idjit. That little thing is the check valve, huh? The Bentley even calls it the check valve. I always called it a relief valve as it let the fluid come back into the reservoir. So how much residual pressure is left in a single circuit VW bus system? On the '67 dual circuit masters the check valve is not in the bore whereas in the single circuit the check valve is in the bore.

Can I still come into the deep end once in a while? I promise I won't splat chlorine water in your eyes or do cannon balls. Please?

:lol: Nobody knows everything. I still get blindsided by people posting stuff I didn't know all the time. :lol:

Wed Mar 24, 2010 7:04 pm

so does this 71-79 master cylinder have these check valves in place even though it was made for disc brakes?

Wed Mar 24, 2010 7:15 pm

BarryL wrote: I even took one to bed with me.

:shock:

I thought about putting my crank start pulley nut on the night stand...... :lol: :wink:

Sat Jun 12, 2010 10:48 pm

sventinker wrote: so does this 71-79 master cylinder have these check valves in place even though it was made for disc brakes?

wow quoting my own post. does any one know the answer to this question?

I went to a local ecology auto yard and found a plethora of rabbit reservoirs and would like to finalize my d/c m/c conversion. Thanks guys

Sun Jun 13, 2010 7:00 am

I don't know if it has the valves specifically like the non disc masters. I do know that the disc specific master cylinders are made with appropriate residual pressures to compensate for the disc brakes. I am not sure how it is done (original style valves in non discs or otherwise, but I know that is the way they are.) The pressure may be vehicle specific (weight wise) so I think that would be the thing to be concerned about.

Sun Jun 13, 2010 8:50 am

sventinker wrote: sventinker wrote: so does this 71-79 master cylinder have these check valves in place even though it was made for disc brakes?

wow quoting my own post. does any one know the answer to this question?

Snoop Bob's right that it will be set up correctly for disc/drum application. When using in a drum/drum application, it will work just fine, it's just over time, the pedal will slowly start to get spongy. To prevent this, you can add the check valve designed for drum brakes.

http://www.summitracing.com/parts/WIL-260-3279/

Sun Jun 13, 2010 6:28 pm

Andrew wrote: sventinker wrote: sventinker wrote: so does this 71-79 master cylinder have these check valves in place even though it was made for disc brakes?

wow quoting my own post. does any one know the answer to this question?

Snoop Bob's right that it will be set up correctly for disc/drum application. When using in a drum/drum application, it will work just fine, it's just over time, the pedal will slowly start to get spongy. To prevent this, you can add the check valve designed for drum brakes.

http://www.summitracing.com/parts/WIL-260-3279/

so one inline for the rear and one for the front?

Sun Jun 13, 2010 6:35 pm

Sven...What master are you going to use? If it was originally a disc/drum master, then you should not need anything.

Sun Jun 13, 2010 7:25 pm

I wanted to use the 71-79 bay mc

Wed Jun 16, 2010 2:49 pm

sventinker wrote: Andrew wrote: sventinker wrote: sventinker wrote: so does this 71-79 master cylinder have these check valves in place even though it was made for disc brakes?

wow quoting my own post. does any one know the answer to this question?

Snoop Bob's right that it will be set up correctly for disc/drum application. When using in a drum/drum application, it will work just fine, it's just over time, the pedal will slowly start to get spongy. To prevent this, you can add the check valve designed for drum brakes.

http://www.summitracing.com/parts/WIL-260-3279/

so one inline for the rear and one for the front?

The master cylinder should have one internally for the rear circuit. You'd just to run one inline on the front circuit.

Wed Jun 16, 2010 3:07 pm

awesome. I now have a great understanding of what I will need. thanks everybody... again

Sat Aug 14, 2010 7:11 pm

Can we get a final concensus on the dual m/c conversion? Reading all this just confuses me.

To convert a 64 bus with single circuit, front & rear drum:
1) 71~79 bus dual circuit m/c
2) Add a check-valve from summit racing to the front circuit (10psi).
3) Spacer and longer bolts needed? If yes, who sells one?
4) Modify push rod?
5) Best fit, easiest to find, lowest cost reservoir =?

Bring it full circle. Thanks

Sat Aug 14, 2010 8:43 pm

wolfgangint.com sells a kit to put a 71-79 mc in a 67 or earlier bus.

Sat Aug 14, 2010 9:25 pm

sheaboys wrote: Can we get a final concensus on the dual m/c conversion? Reading all this just confuses me.

To convert a 64 bus with single circuit, front & rear drum:
1) 71~79 bus dual circuit m/c
2) Add a check-valve from summit racing to the front circuit (10psi).
3) Spacer and longer bolts needed? If yes, who sells one?
4) Modify push rod?
5) Best fit, easiest to find, lowest cost reservoir =?

Bring it full circle. Thanks

1. Yes.
2. Never used those.
3. No, Yes.
4 Or cut a bolt down and round off the end like a push rod.
5. 1967 bus, Rabbit or Volvo, Rabbit or Volvo in an inexpensive wrecking yard.

Sat Aug 14, 2010 9:33 pm

kingodirtp3 wrote: wolfgangint.com sells a kit to put a 71-79 mc in a 67 or earlier bus.

Yes, but the WG kit is $45 + shipping and includes a rubber boot for the m/c. There is almost the identical kit in the ads for $25 shipped, but without the rubber boot. Options, options! :wink:
http://www.thesamba.com/vw/classifieds/detail.php?id=1002657

Sun Aug 15, 2010 7:02 am

I made my own spacer... went to home depot and bought a piece of 1/8" aluminum.... cut it and stacked 4 pieces... then drilled a 1.5" hole through it. Then drilled the holes for the bolts to go through. Worked out where I did not have to adjust my pushrod and I reused the rubber boot off the old single circuit.

no check valve.... running drums on front... no problems.

Cost $14 + time