[kleen56]

I'm running four wheel disc on my truck. The front are standard GM calipers and the rear are 85 Cadillac calipers. I'm running a GM Master cylinder right now, but found that it was for a disc/ drum application and the rear calipers are not functioning due to the lack of volume from the master cylinder. I need a disc/ disc master cylinder. Anyone know which master cylinder would work? I need a manual application with the deep plunger. Any recommendations on piston size? 1" 1 1/8"? What year is commonly used with disc/ disc applications. Thanks guys.

[302 Z28]

The most common one used by street rodders is the 67-69 Corvette 427 435hp four wheel disc brake MC. It can be had in either power or manual. This is one example.

Vince

[Frisco]

Originally Posted by kleen56

I'm running four wheel disc on my truck. The front are standard GM calipers and the rear are 85 Cadillac calipers. I'm running a GM Master cylinder right now, but found that it was for a disc/ drum application and the rear calipers are not functioning due to the lack of volume from the master cylinder. I need a disc/ disc master cylinder. Anyone know which master cylinder would work? I need a manual application with the deep plunger. Any recommendations on piston size? 1" 1 1/8"? What year is commonly used with disc/ disc applications. Thanks guys.

I used the following. Master cylinder is a 1 1/8" bore dual master. I believe it is a 1977-1982 Corvette Power brake master. Delco 18M91 GM #18030333.
I also had 4:1 pedal ratio and a 7" dual diaphragm booster.

The same master cylinder with a deep pushrod hole and a 1" bore (different part #) can be used for non-power use. Pedal ratio will need to be 6:1 minimum with 7:1 being best.

Both front and rear reservoirs will be the same size.

[kleen56]

Thanks guys, that's what I was looking for. I'm running a 8 inch booster our of a 64 Buick Special (that what came with it when I bought it!). The GM master cylinder bolts us to that nicely, but for whatever reason, I have to use a manual master cylinder with it because of the long pushrod on the booster. I have no idea what pedal ratio I have???? Anyone recommend the master cylinders on ebay?

[kleen56]

I checked out the Corvette MC's. I noticed the ones for power brakes come with a bore size of 1 1/8". The manual ones come with 1.000". I have to use the manual MC for my booster. Is there a big difference between the 1 1/8" and 1.000" bore size? Will I be OK with the 1"? Any help is appreciated.

[barnym17]

It will probably be ok but if able to use the larger bore cylinder it will provide more volume at lower pressure which the booster should make up for.

[Triaged]

Why start a new thread and muddy the waters
Disc brake Master cylinder

What exact (or as close as you can get) year/make/model/option are you running for:
1) Front Calipers
2) Front Rotors (or dimensions)
3) Master Cylinder
4) Booster
5) Rear Calipers
6) Rear Rotors (or dimensions)

Is your e-brake hooked up? Does it hold well? How far down does the e-brake pedal go? Are you running any other valves in the brake lines other than the adjustable proportioning valve (like a stock combination valve)? Is the vehicle drivable other than the rear brake problem? If so how far down does the pedal go for an average stop? How much pedal effort is required for an average stop?

[kleen56]

Originally Posted by Triaged

Why start a new thread and muddy the waters
Disc brake Master cylinder

What exact (or as close as you can get) year/make/model/option are you running for:
1) Front Calipers
2) Front Rotors (or dimensions)
3) Master Cylinder
4) Booster
5) Rear Calipers
6) Rear Rotors (or dimensions)

Is your e-brake hooked up? Does it hold well? How far down does the e-brake pedal go? Are you running any other valves in the brake lines other than the adjustable proportioning valve (like a stock combination valve)? Is the vehicle drivable other than the rear brake problem? If so how far down does the pedal go for an average stop? How much pedal effort is required for an average stop?

Oh boy! Let me see if I can answer this.
Front calipers - 74 Monte Carlo GM calipers
Rotors- 11" F150 rotors.
Master cylinder- GM Disc/drum- manual
Booster- 8" 64 Buick Skylark special
Rear calipers- 1985 Cadillac Seville
Rotors- Cadillac rotors. 11"
Ebrake hold the vehicle and I'm using Lokar cables. E brake is a handle not the foot pedal type. I get about 2-3 clicks max. I am only running a Wilwood adjustable proportioning valve which is currently wide open. The front brake work great, and is drivable. The pedal is firm and drops about a quarter of distance. Basically, the brakes work fine, just not utilizing the rear brakes when the brakes are applied or very little if any. I ordered the Corvette MC for disc/ disc and hoping that will cure my rear disc problem. Any other suggestions.... I'm listening! LOL....

[Triaged]

I really don't think your problem is the master cylinder (at least replacing just your MC won't fix your problem). Your manual brake master cylinder most likely has a 1" bore vs the normal 1.125" bore for power brakes. However your booster is small so having the smaller master should balance it out ok.

Right away I see one thing. The front calipers are off a full size, while the rear calipers are off something that used smaller "metric" front calipers . So right off the bat you have some extra front brake bias. I'm not a big fan of calipers with the e-brake integrated. They often cause problems like you are having, however it seems like you have gotten the e-brake part taken care of.

In 83 GM started using low drag calipers that retracted the pistons. To make up for this they used a step-bore (aka quick take-up) master cylinder. I don't know what they did to the rear calipers in this regard but it sounds like you might have a mismatch there too.

You turned the prop. valve clockwise all the way in...right?
If you put say 1 click on the e-brake handle will the rear brakes work off the hydraulics any better? Can you pump up the brakes and get the rear brakes to hold better? If so a 2# residual pressure valve in the rear brake line should help.
As for the difference in caliper size you might look into using high friction pads in the rear while using low friction pads up front. Say an EE front and FF or GG rear. Basically a performance pad in the rear and a jobber/fleet pad up front. The parts guy will have to pull the box and you will have to look on the edge of the friction material...I don't think the edge code is in any parts book or online.

edit
If you want to change the master cylinder I would recommend going with a small 83-up double diaphragm booster/step-bore master combo and changing out your calipers to 83-up full size calipers. That would get you a light aluminum master along with the low drag calipers which would be good for both MPG & MPH.
/edit

From: http://www.satisfiedbrakes.com

5. What do edge codes mean?
An edge code is a means of identification that may be used to describe the initial frictional characteristic of any brake lining.

Typically, a two-character code (e.g. EE, FF, GG, HH, etc) is used on specific friction formulation. These characters represent the coefficient of friction when a 1" square piece of friction material is subjected to varying conditions of load, temperature, pressure and rubbing speed on a test apparatus known as the Chase machine.

The coefficient of friction measured by the Chase test describes the relationship between the two forces acting on the friction material. A clamping force is exerted on the friction material, resulting in a frictional or resistance force. A low coefficient of friction means that very little of the clamping force is transferred into resistance force. On the other hand, a high coefficient of friction means that given the same level of clamping force, a higher resistance force is generated by the brake pad.

For example, a pad that carries an HH code has a normal coefficient of friction of 0.55 or higher, and a hot coefficient of friction of 0.55 or higher.

The first letter of the code represents the normal friction coefficient. This is defined as the average of four test data points measured at 200, 250, 300 and 400 degrees Farenheit.

The second letter of the code represents the hot friction coefficient based on a fade and recovery test. We all should know what brake fade is. If you've ever had to use the front brake extensively and found that its effectiveness quickly diminished, that's fade. Recovery is basically the period where the brakes are gradually cooling off.

The hot friction coefficient is defined as the average of 10 data points located at 400 and 300F. On the first recovery cycle of the pad; 450, 500, 550, 600 and 650F. On the second fade cycle; and 500, 400, and 300F on the second recovery cycle.

The range of friction coefficients assigned to each code letter are as follows: C = less than 0.15. D= 0.15 to 0.25. E= 0.25 to 0.35. F= 0.35 to 0.45. G= 0.45 to 0.55, and H= over 0.55.

[kleen56]

Excellent source of information. Thank you. This is the information I have gotten from various guys who build hotrods for a living. I understand what your saying and it does make sense. Thanks. I was told, that my MC is a disc/ drum MC. which means the front will work sufficiently with the disc brake on the front end. The rear, which is drum, worked nicely when I was running drum brakes. I was told the drum portion has a built in 10lb residual valve in the MC, at the outlet. In addition, the drum portion of the MC will not provide enough pressure or volume to efficiently operate the discs, particularly with full size, Cadillac type calipers. I was told I have to have a 4 wheel disc/disc master cylinder and my brakes will work fine.
I also tried the method you described as clicking the ebrake slighly and then using the hydraulic. There was no difference. The MC is not pushing enough fluid to the rear calipers to push the piston out to the rotor. I also had a problem with the inline taillight switch which is in the line for the rear brakes. A Tee with the switch. With the drum brakes, I had no problem with the taillights functioning. When I converted to the disc in the rear, my taillights no longer functioned. They did come on if I stood on the brake pedal with tremendous force to build pressure in the line. The lights would come on. I wanted to make this easy, but sometimes that doesn't happen. I really don't want to invest in more money in new calipers, etc. I think my first attempt will be to try the disc/ disc MC and see if that works. If not, then I'll switch to different calipers. Or...... I can always reach down and pull the ebrake at every stop. I truly the appreciate the info you gave me and will keep it for future reference. Thanks.

[Triaged]

The problem I was describing was NOT disc/drum vs disc/disc but low-drag vs non-low-drag setups. In fact if your master cylinder had a residual pressure valve in it that would be a GOOD THING! The problem is also not the rear brake calipers being too big but rather too small. From what you posted it was as if you took all I was trying to say exactly the opposite from how I meant it. How much was that new supposedly different by being a "disc/disc" master? Will they give you your money back after you install it and it doesn't help your problem?

I hate those hydraulic brake light switches. A switch on the pedal is the only way to go IMO. I would ditch it along with the prop. valve to eliminate as many variables as possible. Verify that there is no residual valve in the master cylinder while you are at it.

[kleen56]

I already got rid of the hydraulic switch and went with the mechanic on the pedal. Your right the hydraulic is junk. As far as the master cylinder, a friend of mine works in an auto parts warehouse and I can pick one up for $40. If it doesn't work, he'll exchange it. It's not that big of deal switching out the MC either. If that doesn't do the trick, I'll go to your recommendations. Again, thank for the help.