[PITTSBURGH 64]

Can I use my good 67 drum dual master cyl on my disc/drum conversion?
If so, how do I remove the front reservoir check valve?

Are rebuilt masters as good as new ones?

I just went through 2 bad NAPA master cyls. Both of them were new not rebuilds.

Thank you

[engineer]

since you need different reservoir capacities as well as bore size, no.

[Scarebird]

This is not true at all. Most disc masters from our vintage cars share the same bore as the drum masters. Reservoir capacity is pretty much the sole difference. We have had MANY customers with single chamber masters just swap out the drum stuff and put in disc with no ill effects- vehicles such as 57 Cheiftains and 64 Rivs.

[chief many horses]

Pit--64
I've never done it but if you do decide to go that way just reach in the port, where the front line is installed,with a wire etc. and pull out the "residual valve", (It may be screwed in-forgot). The valve is installed to keep approx. 3-4# of pressure on the drum brake cyl's to keep them from leaking down. Discs don't need them. Not too sure if the lesser capacity will work but I feel that the Sarebird is correct about the earlier MC's. Let us know how it works~

[scottk]

If the valve is in there you should be able to feel it. If it is just ruptured it could hinder fluid flow if it gets cockeyed in there. Ruptured or not it could create drag on the discs. I have not found any RPVs when I've checked, but they are listed for 1967, and I've not checked an OEM 67 cylinder.

You have to remove the seat from the outside to remove the valve. I ran a tap thru mine and then a bolt to press it out, according to service manual, but then had to replace the seats. This was just for cleaning, there were no check valves in it.
I've read where some people screw a sheet metal screw into the seat hole and then either slide hammer or forcefully pull it out, so they could re-use it. New ones come with some rebuild kits.

AFA disc vs. drum masters...
Bore hydraulic pressure is proportional to pushrod apply pressure minus the resistance offered by the piston return springs inside the master. The different springs and piston lengths/tensions in different masters can effect operation, even if the bore diameter is the same. Front and rear port pressures are rarely exactly equal. I would use the correct master, but the incorrect one might work OK for you.

[JUDGGTO69]

Pitt you are going through all the BS that I was goin through thats why I ended up with my electric boost setup!!!!!!!!!!!!!!!

[sts]

They are not interchangable. Drums use a 1" bore and discs use 1 1/8" bore. The different size bore creates a different hydralic value. Just because you can stop the car with the wrong master cylinder, doesn't mean it's the correct m/s. If you were to try the incorrect m/s then the correct one, you would see a huge difference in brake performance.

Reservior capicity has zero to do with braking. If the master cylinder has enough fulid so that it doesn't suck air, you will have the same braking power that you would have with a topped off reseservior. The reservior is as its name imples. It's additional fluid so that you have extra fluid in case of a leak.

[awsum67]

Since we are sharing information and experiences about disc vs drum brake master cylinders I would like to share an experience that was triggered by Chief Many Horses and his comments about the "residual pressure valve" (RPV).

About 12 years ago I repacked all of the wheel bearings on my 66 Corvette. During the process I also installed a complete stainless steel braking system from Vettes Brakes & Products out of FL. I did not know it at the time, but they shipped me a Chevelle drum brake master cylinder rather than the correct one for my car.

Whenever I work on wheel bearings and start driving the car again, I always touch the wheels whenever I stop to be sure they are not running hot on me. Its just a habit formed back when my Dad first taught me how to repack wheel bearings. After it was back on the road, I did check the wheels, and all four were hot enough to seriously burn your hand.

Long story short, the RPV was forcing me to drive with my disc brakes applied. Running 75 miles an hour in a Corvette with your brakes on generates a lot of heat.

Vette Brakes told me to send it back for a refund, but they did not have anymore disc brake master cylinders to replace it with. I could not find a correct one locally so I removed the RPV and the problem went away. It was not very difficult, just the aggravation you have to go through whenever you change any master cylinder. Once the wheel temperatures went back to normal, I never worried about the master cylinder bore issue. It just felt good that my tires only smoked when I started but not when I stopped.

What I learned was to never assume that disc and drum master cylinders are interchangeabe. After jumping that snake, I always do my homework.

[chief many horses]

Interesting-- Imagine it's happened many times with guys going crazy wondering what went wrong!