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Disc brake Master cylinder

9K views 19 replies 8 participants last post by  Triaged 
#1 ·
I recently got rid of the drum brakes on all four corners. I replaced the brake lines, etc. I purchased a GM Master cylinder (the one with larger reservoirs for front and rear), I was told it was a Corvette MC. The front disc work great, the rear seems to not have enough pressure to push the piston on the caliber enough to stop the vehicle. I'm using 85 Cadillac calipers on the rear. I was told that some MC's came for a disc/drum setup, and if you ran disc on the rear, there was some type of seal, or restrictor on the MC that could be easily removed (I believe at the outlet of the MC. Does anyone know about this and do I need to remove it for rear disc brake applications.
 
#2 ·
Generally a disc MC will have a different bore size than a drum MC. You say your MC is for a Corvette. The early Vettes came with either Disc front, drum rear or all disc. They also were available with or without power booster which is a different MC. Without knowing just which MC you have I cannot make any recommendation.

To answer you question however, I am not aware of any "quick fix" to solve your problem. Question: do you have an adjustable proportioning valve on your vehicle, and give us some specifics about your car?

Vince
 
#3 · (Edited)
Thanks Vince, I'm not sure what year this MC came from. When I purchase it new, I told the guys I had a GM booster and was planning on running disc on the rear, but right now will be running drums for a while. The disc brakes are on my 1956 Ford F100 truck. The disc on the front was a conversion kit from CPP and they use GM calipers. The rear calibers on a Ford 9 inch rearend are from a 85 Cadillac because they come with the ebrake levers for easy ebrake hookup (don't recommend these calipers to anyone, they are a pain to set up). I am running a Wilwood adjustable proportioning valve in the line going to the rear brakes only. I even set it to wide open and it helped but not much. I am running a booster from a 63 Buick Special (don't ask!) :mwink: Initially I was told that this MC would work for either drum or disc applications as long as I had the proportioning valve in the line for drum application. The MC is also mounted on the firewall and not on the frame. Now I'm running disc in the rear and it seems like there isn't enough pressure coming to the calipers. They bleed OK and I have a firm pedal, etc. I know disc require more pressure then drums. A friend who builds streetrods told me there is small diaphram or something that restricts pressure on the outlet of the MC for drum brake applications. He said that is the only difference between the disc and drum master cylinder. If you remove that, you will have the same pressure as the front for disc brakes? Before I remove the lines, etc, I'm wondering if this true? or open to suggestions.
 
#4 ·
There is no difference between a GM disc/drum and a disc/disc master cylinder! The difference is in the combination valve (which it sounds like you aren't using) and type of assist (manual, vacuum, hydraulic). Those e-brake calipers (all of them) are junk. Everyone that I have ever talked to that has used them has had the exact same problems as you are having.
 
#5 ·
Unless it is a really new corvette master cylinder, with the integral prop valve, there is nothing that restricts the pressure to the rear. The master cylinder with the integral prop valve is easily identifed by an extra casting on the side of the master cylinder body that the line outlet to the rear is attached..

Other than that, in a drum brake master cylinder outlet there is only the residual pressure valve spring and under the brass seat ...That does not restrict brake pressure.

You could have bought one of the new master cylinders with the integral prop valve, but even if you have one of those, with the adjustable one turned down, there should be no problem.

You would do well to get a brake pressure gauge, hook it up at the calipers, check the brake pressure at the caliper bleeder port...That will help you get to the root of the problem. Anything less than 1100 psi at the caliper when the pedal is pushed as hard as possible will give poor performance with the newer brake pads.

Later, mikey
 
#7 ·
Triaged said:
There is no difference between a GM disc/drum and a disc/disc master cylinder!
Ok i need an education on this one... In 30 years of working on GM cars i have never seen a disc/drum master cylinder that was the same as a drum/drum master cylinder.

The disc/drum master cylinders have a larger area for the increased capacity needed by the calipers over the wheel cylinders.

The combination valve is different for the disc/drum.

Keith
 
#8 ·
I think triaged might mean that internally there is no difference.

Some of the aftermarket guys sell master cylinders that they call "corvette" style masters, with 2 big reservoirs.

The ones I've seen for disc/drum have an RPV in the rear brake line outlet, installed under the brass seat in the outlet.

I always wondered about the springs that are in in between pistons , if they are different from each other in a drum/disc ,disc/disc or drum/drum master, but I can't find any info on them..They are so weak compared to the pressure that is inbetween pistons that I can't see them making a big difference.


Later, mikey
 
#9 ·
kleen56 said:
I recently got rid of the drum brakes on all four corners. I replaced the brake lines, etc. I purchased a GM Master cylinder (the one with larger reservoirs for front and rear), I was told it was a Corvette MC. The front disc work great, the rear seems to not have enough pressure to push the piston on the caliber enough to stop the vehicle. I'm using 85 Cadillac calipers on the rear. I was told that some MC's came for a disc/drum setup, and if you ran disc on the rear, there was some type of seal, or restrictor on the MC that could be easily removed (I believe at the outlet of the MC. Does anyone know about this and do I need to remove it for rear disc brake applications.
You may try reversing the lines (out of mc).
 
#10 ·
powerrodsmike said:
I think triaged might mean that internally there is no difference.

Some of the aftermarket guys sell master cylinders that they call "corvette" style masters, with 2 big reservoirs.

The ones I've seen for disc/drum have an RPV in the rear brake line outlet, installed under the brass seat in the outlet.

I always wondered about the springs that are in in between pistons , if they are different from each other in a drum/disc ,disc/disc or drum/drum master, but I can't find any info on them..They are so weak compared to the pressure that is inbetween pistons that I can't see them making a big difference.


Later, mikey
I think Mikey is describing the RPV in the rear brake line outlet, is the item that my friend is talking about. I don't know if that is removable or if that makes a difference as far as pressure in the line. My gut feeling is the calipers that I'm using are the problem. I know I had a hell of time just trying to get the clutch to work and have a e brake that works. I'm using Lokar ebrake lines which really made the job easy. I recall when I had the drum brakes in the rear, I was using a hydraulic brake light switch that spliced into the same line as the rear brakes. When I went to disc brakes on the rear, I noticed the brake lights didn't work unless I really stood on the brake pedal. I eliminated the switch and went with a mechanical switch. Apparently the disc brakes made a change to the pressure in the line causing me not have brake lights. I really appreciate your help guys, but now I'm more confused then ever. :confused:
 
#11 ·
You can remove the valve at the outlet. It is not needed for disc brakes. Removing it will not change the output pressure at all. The only purpose of that valve is to keep some hydraulic pressure on the line to keep the seals in a drum brake master cylinder seated.

To remove the rubber valve, tap into the small hole in the brass inverted flare seat with a 6-32 tap, then take a 6-32 screw, about 1" long with a 6-32 nut and a washer bigger than the outlet, screw the nut all the way onto the screw, then put the washer on, then screw the screw into the tapped hole.

Tighten up the nut and it will pull the seat out. The valve will come out with a pair of tweezers. Get the spring out also.


It won't fix your problem with low brakes, but you won't have dragging brakes.

Is that system bled? How far does the pedal go before it gets firm?

What calipers are on the front? what bore size is your master cyl?

Later, mikey
 
#12 ·
It won't fix your problem with low brakes, but you won't have dragging brakes.

Is that system bled? How far does the pedal go before it gets firm?

What calipers are on the front? what bore size is your master cyl?

Later, mikey[/QUOTE]

My brakes aren't dragging now, so maybe all that effort isn't worth it. Thanks for the info, I may still use it. The system is bled, and I have a good pedal. The pedal goes, hard to say without measurement, but I'm guess a quarter of the way, maybe 3/4". The calipers on the front are GM calipers, as far as bore size, I was told they are 1 1/8". My brake pedal doesn't fade and the truck stops nicely, but I'm primarly using the front brakes only and I can tell the rear rotor isn't getting my brake pad action. I could go with GM calipers like the front, but then I forfeit the use of an e brake (which would be nice to have. )
 
#14 ·
I've been adjusting these calipers every which way possible. I'm not sure if I have a 1/4" air gap, I do know that I have 4-5 clicks of the brake handle. The e brake is working but probably not as well as it should. I may have to use the click method and do it 30-75 times. Geez, you'd think there was a better method. Whose the Einstein that designed these calipers? I may bite the bullet and use standard calipers and no utilize an ebrake. Thank you for the great info. You've all been very helpful.
 
#15 ·
Yeah those E-brake rears are more of a parking brake than an emergency brake. My Willwood rear discs on my 34 have the integral E brake and it's not worth a flip for stopping, but it works fine for a parking brake. Now for some reason the E brake on my son's Corvette works great and it's the same general arrangement as my 34.

Vince
 
#16 ·
I don't know if this helps or not, but I researched the Master cylinders. According to several sources, if your running 4 wheel disc, you should not have a restrictor in the line or the one that is on the master itself. They said the restrictor is for drum brakes so they stay expanded. The restrictor could cause dragging on a disc brake application. I guess for now, I'll live with what I got, until I can come up with a solution. :sweat:
 
#17 · (Edited)
kleen56 said:
I don't know if this helps or not, but I researched the Master cylinders. According to several sources, if your running 4 wheel disc, you should not have a restrictor in the line or the one that is on the master itself. They said the restrictor is for drum brakes so they stay expanded. The restrictor could cause dragging on a disc brake application. I guess for now, I'll live with what I got, until I can come up with a solution. :sweat:
It is not a restrictor...It is a residual pressure valve, lets fluid flow out with no resistance, the fluid is allowed to flow back in the master with no resistance..until the line pressure drops to approx 10 psi, (2 psi for disc brakes) the valve shuts, and it holds that pressure in the line. It basically takes up the "slack" in the wheel cylinder seals.

Later, mikey
 
#19 ·
Not being a grammar nazi, just helping with the right words here and there... :D


( sometimes I'll be writing a post, scratching my head, thinking...I need a word here....it starts with a t.. or was that a w...p maybe? :D )

I usually remember what word I wanted at 2:30 am..


Later, mikey
 
#20 ·
302 Z28 said:
Quite the contrary, look up the Raybestos listing for 67-69 427 435hp Corvette.

Vince
As with everything I'm sure there are exceptions...but there was no disc/drum master in those years right :confused: ...so that isn't an exception :D It would also tend to back up my claim of the difference being more dependent on type of assist (or other brake option package).

k-star said:
Ok i need an education on this one... In 30 years of working on GM cars i have never seen a disc/drum master cylinder that was the same as a drum/drum master cylinder...
I was just talking about disc/disc vs disc/drum and not drum/drum. Given the same calipers/rotors up front and just going from drum to disc in the rear GM would use the same MC and just mess with the combination valve.

No?
 
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