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Snoop Bob Wed Aug 10, 2011 6:08 pm

Will anyone that has EXPERIENCE (not theory) with a 67 only master and front disc kit please post. The question is which insert (valve) if any is removed/unscrewed from the master before installing the line when using front disc brakes and rear drums.

A. The Front
B. The Rear
C. Leave them it doesn't matter

A secondary question would flow into when using front and rear discs. Do you remove them both or leave them both?

Thanks in advance.

type241 Wed Aug 10, 2011 8:10 pm

Contact Russell @ OLDSPEED (562)531-4190.

Andrew Wed Aug 10, 2011 10:23 pm

Ok, if you are running front discs and rear drums with a '67 only master cylinder, unscrew the residual pressure valve from the FRONT circuit of the master cylinder and use a pair of pliers and just yank out all the guts from it and then screw the valve back in so that you are more or less using it as an adapter.

If you have front and rear discs, then you need to yank out the guts from both FRONT and REAR residual pressure valves on the master cylinder.

The reasoning behind this is that drum brakes need somewhere around 75 lbs of residual pressure, which is provided by the residual pressure valves (and then counteracted by the return springs in your drums). Disc brakes need very little, somewhere in the neighborhood of 5-10 lbs, just to keep the pads against the rotor. With no residual pressure (which is what happens when you yank out the guts), the pads won't stay as close to the rotor as they were designed to be; but that just means that when you hit the brakes, instead of the pad instantly pushing against the rotor, it will have to travel a small fraction of an inch first. More or less like your calipers are self adjusting every time you hit the brakes. Most people won't even notice it. On the other hand, if you leave the stock residual pressure valves, your disc brakes will function normally, but it would be like driving around with the brakes on all the time.

nelsnfam Thu Aug 25, 2011 10:37 am

I agree with Andrew's description above. Although I believe the residual pressure valves only retain 10 lbs pressure not 75 lbs.

Be that as it may, it's still like driving with your foot on the brake all the time.
Here are some photos of the procedure.









Hope this helps 67 only disc brake conversions.

Steve Nelson
http://ratropa.com

Andrew Thu Aug 25, 2011 3:57 pm

nelsnfam wrote: I agree with Andrew's description above. Although I believe the residual pressure valves only retain 10 lbs pressure not 75 lbs.

Be that as it may, it's still like driving with your foot on the brake all the time.
Here are some photos of the procedure.

Hope this helps 67 only disc brake conversions.

Steve Nelson
http://ratropa.com

Great pictures of the procedure! :D

nelsnfam Thu Aug 25, 2011 4:16 pm

Thanks Andrew. I took it apart slowly and carefully to get some decent photos.
If I were doing it again, I'd just squeeze the business end in a vice and rip it off.
Anyway, I took it for a test drive and things were just as you said they would be. Brakes stay cool, pedal feels normal and it stops great.

Thanks for the advice. It gave me the confidence to get in there and do it. We don't need no stinking residual pressure.

Steve Nelson

Snoop Bob Thu Aug 25, 2011 9:07 pm

Let me ask this. Would the line not screw directly into the master after removing that screw in assembly, or is it a different thread pitch or size? Hoping to do this, this weekend.

nelsnfam Fri Aug 26, 2011 6:38 am

Snoop, no, the line will not screw into master. The line fitting diameter is smaller.
Also the line would need to be bent or reshaped to line up properly with the opening in the master cylinder.

As Andrew pointed out, the gutted residual pressure valve acts as a spacer/adapter for mating the line to the MC.

If you were hoping to salvage the residual pressure valve functionality so you could revert back, if you weren't happy with no residual pressure, here are some ideas.
1. VERY carefully remove the bits and pieces of the valve so you can put them back on. The casing that holds it all together is just crimped to the fitting.
2. Find a used 67 MC and salvage the valve from it.

Hope this makes some sense to you.
I really think you will find front discs on a split bus a better driving experience with no residual pressure.

Steve Nelson

Snoop Bob Fri Aug 26, 2011 1:25 pm

Thanks. Just trying to cut out a step, wasn't really concerned about the line as I am bending up a new one. Thanks for posting that tech pic tutorial.

nelsnfam Fri Aug 26, 2011 2:20 pm

Also, the fitting has the correct internal shape to mate up with the flare on the end of brake line.

Besides thread size and pitch, flare matching is important too.

Steve Nelson

Snoop Bob Fri Aug 26, 2011 3:42 pm

nelsnfam wrote: Also, the fitting has the correct internal shape to mate up with the flare on the end of brake line.

Besides thread size and pitch, flare matching is important too.

Steve Nelson

Thanks, I am aware, It appears the residual valve flare matches the standard double flare on the brake line

brettsvw Sat Aug 27, 2011 6:48 am

When I was in school a free brake class was offered and this is what I got from the class. I am not a writer so I hope I make this understandable.

Disc brake calipers work only because of the specific design of the square-cut seal and design of the groove the square-cut seal sits in.



Here is a pdf drawing of a caliper sliced in two.

http://www.ucx.com/documents/squareseal.pdf

The seal grips the piston and only allows the piston to slide on the seal when the pads wear. The groove that the square cut seal sits in has a beveled edge on the side towards the pads that allows the seal to twist as pressure is applied to the piston . When pressure is released the seal being twisted wants to return to its proper molded position and this moves the piston back away from rotor just enough to separate pad from rotor (no springs to do the job like drum brakes).

By leaving the residual valve in you are keeping pressure applied and this is incorrect. Eventually the seal will loose its grip on the piston and always keep pads touching rotors and premature wear will be the result.

Thea cause of caliper failure is when the seal does not grip the piston and the twisting action of the square-cut seal is no longer present and will no longer return piston like springs in drum brakes release shoes.

I hope I did not confuse anyone.

Andrew Tue Aug 30, 2011 9:40 pm

nelsnfam wrote: If you were hoping to salvage the residual pressure valve functionality so you could revert back, if you weren't happy with no residual pressure, here are some ideas.
1. VERY carefully remove the bits and pieces of the valve so you can put them back on. The casing that holds it all together is just crimped to the fitting.
2. Find a used 67 MC and salvage the valve from it.

Another option is that aftermarket residual pressure valves are available from companies like Wilwood. It requires cutting the metal line in a spot and putting some fittings and bubble flares on the ends, as the valve goes in line rather than screwing into the master cylinder; but it shouldn't be too difficult with the correct tools.

vinlong Tue Apr 29, 2014 11:58 pm

Another option is that aftermarket residual pressure valves are available from companies like Wilwood. It requires cutting the metal line in a spot and putting some fittings and bubble flares on the ends, as the valve goes in line rather than screwing into the master cylinder; but it shouldn't be too difficult with the correct tools.[/quote]


I'm looking into doing what Andrew said, as its probably the best way to add RPV's, but the Wilwood/Speedway (actually everything I've found) RPV's have tapered NPT pipe thread at each end, and its a PITA to find adapters that go from metric bubble to NPT, so I'm thinking about cutting the line, flaring the ends with standard SAE double flare, THEN I can use SAE to NPT adapters.

2true Wed Apr 30, 2014 4:42 am

type241 wrote: Contact Russell @ OLDSPEED (562)531-4190.

x2

55samba Wed Apr 30, 2014 5:32 am

When I did my 944 disc conversion back in 05 I used a bug MC because it had the smallest piston I could find. Then I installed 2 psi residual valves in the lines. There was no friction on the pads but they only came back far enough to release. The residual valves were about 1 inch long and threaded into the MC and the rake lines fit right into them. Without them there was way to much free play with a small piston MC. The smaller your MC piston the stronger your brakes will be.

Something like this.

http://volks-shop.com/2-psi-metric-residual-valve-...-5497.html

vw nutter Sun Jul 25, 2021 1:32 pm

does anyone else have actual experience with running disc front and drum rear in a split bus?

what is the best master to use?
- Baywindow bus master?
- 67 bus master?

what is the best combo?
any tricks?

also what bias valve is needed? or none?

srfndoc Sun Jul 25, 2021 1:52 pm

The chinese copy 67 master/reservoir is the way to go.

chrisflstf Sun Jul 25, 2021 4:40 pm

x2 on the above

vw nutter Mon Jul 26, 2021 11:07 am

ok,
that has been my assessment so far to go with the 67 Master

but only 2 options of brands & quality that i can find so far;

option 1, Chinese unknown brand for $50-70
or
Option 2, ATE original German quality $200-300

any other options for a 67 master??



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