[35olds]

Has anyone been successfull using a 1988 corvette master cylinder? I am using Mustang II disc brakes up front and G.M. drums in the rear.My pedal is soft.I've done all the tests .Everything checks out.Mayby the 7/8" bore is too small for my set-up? I've purchased a new cylinder with a 1" bore,once I try it ,I can't return it.Thanks for any help!35OLDS Also ,it is a firewall mount.I even isolated the brake ,blocking off the rear,switching the lines still spongy and almost to the floor b-4 it locks up.Thanks again!! they are manuel brakes

[302 Z28]

The most common m/c used on street rods is a 67 Vette. Why did you settle on an 88?

Vince

[joe_padavano]

Why would you use a disc/disc master cylinder on a disc/drum braking system?

[dawg]

ya cant do that?

[redsdad]

Originally Posted by dawg

ya cant do that?

You have to put a 10 lb. residual pressure valve in the line to the rear drum brakes. Wheel cylinders only seal one way and can allow air into the system under certain conditions. The square cut o-rings on disc pistons seal both ways and don't require a residual pressure valve (unless the master cylinder is located below the calipers, like under the floor of a hot rod. Then you need a 2 psi residual pressure valve in the disc line to keep the pistons from retracting).

[Bryan59EC]

Originally Posted by joe_padavano

Why would you use a disc/disc master cylinder on a disc/drum braking system?

nice catch-----went over my head the first time I read it----

Bryan

[35olds]

Originally Posted by 35olds

Has anyone been successfull using a 1988 corvette master cylinder? I am using Mustang II disc brakes up front and G.M. drums in the rear.My pedal is soft.I've done all the tests .Everything checks out.Mayby the 7/8" bore is too small for my set-up? I've purchased a new cylinder with a 1" bore,once I try it ,I can't return it.Thanks for any help!35OLDS Also ,it is a firewall mount.I even isolated the brake ,blocking off the rear,switching the lines still spongy and almost to the floor b-4 it locks up.Thanks again!! they are manuel brakes

Well,first of all,all 67 Corvettes had 4 wheel disc brakes,2nd,the 88 cylinder is very attractive looking compared to most,all my research tells me that a 4 wheel disc cyl. will work on a rear drum set-up. I plugged the rear brakes off,and the front is still spongy,the pedal travel is way too much B-4 the brakes lock.I guess I'll try the larger bore cyl., thanks

[redsdad]

Spongy pedal and too much travel can be separate problems.

Spongy is most commonly caused by air. But is can also be caused by bad hoses or an improperly braced master cylinder mount. Is this on the 35 Olds your name implies? Watch the firewall while someone pushes the pedal. Are the calipers mounted so the bleeders are at the high point? (I know that question may seem stupid, maybe even offensive, but you would be surprised at how simple that is to overlook.) Are the hoses new and/or in good condition? When you bled the brakes, did you gravity bleed them? If you had someone pump the brakes, did they do it gently or did they rapidly ram the pedal and aerate the fluid.

Once you are sure the system is good, you can look at pedal travel and pressure. A small bore master cylinder will require more stroke to move the same amount of fluid as a larger bore cylinder. A larger bore master cylinder will require more pedal force to produce the same line pressure as a smaller bore cylinder. So you have to size the MC accordingly.

You can find a lot more info here:

http://www.mpbrakes.com/technical-support/reference.cfm

[35olds]

Thanks REDSDAD ,that site is the one I use the most,when it comes to brakes.I even called the guys there but ti no satisfaction, To bleed the brakes I had the wife to help,YIKES,and told her to slowly depress pedal.I've had her help me numerous times bleeding brakes.The fire wall mount is very solid as to how it is hooked up.The pedal swings from the same solid mount that the cyl. is mounted to.The !" bnore may help,but ,I'll loose pressure.I'm about 6.4 to 1 so I'll have plenty of leverage. Thanks again, 35 Olds.,,,Yes I'm building a 1935 OLDS 3 Window coupe, Yes all 35 Olds coupe were 3 window, as were the pontiacs,you could get chevy in 3 or 5 widow units.

[Timmijoe]

did you bench bleed the master cylinder before you put it on the car, 35olds?

[35olds]

Yes I did ,that is standard procedure,yhanks,35olds

[Triaged]

Sounds like you have air in the lines to me...not too small of a MC.

[427v8]

too small a MC or too much pedal ratio can also make it feel soft.

I don't think you can use a disk/disk with disk/drum. drum brakes need much less pressure or something. I really think you need a diff MC

Using Mustang II brakes should be a known issue, what did MP bakes recommend?

[redsdad]

Originally Posted by 427v8

I don't think you can use a disk/disk with disk/drum. drum brakes need much less pressure or something.

You're right about the pressure issue. However, the MC is just a pump. There are 3 main differences between a disc/disc and a disc/drum MC and peripheral equipment.

A master cylinder for a disc brake system usually has a larger reservoir for the disc brake circuit. A larger reservoir used for drums will not cause a problem.

Drum brakes require a residual pressure valve as I stated in an earlier post. You can purchase external residual pressure valves from several sources. Not using a residual valve on the drums could allow air to enter the drum hydraulic circuit. However, 35Olds says the rear is disconnected and blocked, so this shouldn't be an issue.

The lower pressure requirements of drum brakes are handled by the proportioning valve which is external of the master cylinder. It can either be a combi valve located under the MC or an adjustable unit mounted in the drum brake circuit.

IIRC, there may have been some dual bore size cylinders where the drum portion had a larger bore to compensate for the lower pressure requirements, but the proportioning valve is a much better solution, IMHO.

I hope you get this sorted out 35olds. Let us know how it is going.