My earlier post outlined the lack of brakes in the rear and the pedal that nearly goes to the floor. (1967 Chevelle, Disc front, Drum rear, power brakes.)

I'm thinking of replacing the Master Cylinder. My MC has a single wire retainer over the top. I notice in a catalog that only Drum brakes have the single wire and that all the Disc brake MCs from '67 to '72 have two wire retainers. Is that true?

Does that mean I have a drum brake MC? And would that cause my brakes to not work properly? The front calipers do engage on the discs.

And is there a difference between the Power Booster for the drum brakes and disc brakes?? Do I need to replace both the booster and MC?

Thanks!!

i dont know what kind of setup you have, but if someone converted it to disk brakes, they may have eliminated the prop. valve-drum brakes need a 6 lb residual valve to them to keep pressure @ the wheel cyls(the springs are strong enough to pull the shoes back) w/o you will get a soft pedal(trying to fill the line & cyl up)
their is a diff bet the 2 m/c's(piston size)

The single bail drum cylinder has equal resivoirs. There are single bail disc cylinders with unequal resivoirs... the disc resivoir is much larger than the drum resivoir on these. Disc/disc or disc/drum masters also tend to have a deeper stroke.

Most power disc masters will be 1 1/8, while the drum master will have a 1" bore. The brakes would still work, but as pointed out, a residual valve on a disc system can cause drag, and the lack of one on a drum system can cause a soft pedal. Likewise, too small of a bore can cause a soft pedal.

The boosters are as frequently the same as they are different. Drums dont realy need power assist, so their boosters tend to be smaller if there is a difference. Often they just run the same one... it depends on the year/model, I think.

The residual valves on early GMs are located inside the master cylinder, behind the seat in the drum port(s).


single bail drum/drum http://partimages.genpt.com/thumbs/304609.jpg1967-72

single bail disc/drum http://partimages.genpt.com/thumbs/33685.jpg1971-80

dual bail disc/drum http://partimages.genpt.com/thumbs/304845.jpg1967-69

dual bail disc/drum http://partimages.genpt.com/thumbs/33640.jpgmanual brake 1970-72

Thanks for the help. The proportioning valve is still there, but whether it is the right one might be another matter. I suspect this car originally came with disc brakes, but can't be sure. The rotors I replaced were worn past the point of any rotors I have ever seen. Certainly not a recent conversion, unless a previous owner bolted worn out junk on.

The master cylinder I bought looks like the "dual bail disc/drum" for 1967-69. The parts store said it was the same number for 1967 and 1970, with a different number for 1968-69.

I hope to try it tomorrow.

like chevry mentioned, most of the early gm cars had the residual check valves for drum brakes behind the seat in the master cyl ports.

BUT, every replacement master ive ever seen from local parts stores DO NOT have these check valves. you will have to remove the residual check valves from an original master and swap them into the new/rebuilt one. in your case you would only need it in the port for the rear.

i mention this because i just went through this myself. recently i swapped to a dual reservoir master on my 66 and the none of the master cyls at any of the parts stores had the check valves in place. luckily i had an old original master laying around and was able to use the residual valves out of it.

if you cant find a master with the check valves in place, you will have to use an external check valve such as the ones that wilwood offers.

also, for some reason the stock single reservoir that i removed off of the 66 didnt have a residual check valve in it. the pedal was always low and you had to pump it to get any sort of firm feeling.

Brandon,

I'm not clear on this residual check valve part and how to check if it is there in the new MC. How would I recognize it? Is it necessary to disassemble the MC to see it or to install one if it does not have one?
Thanks.

northern

its kinda hard for me to explain what they look like so dont razz me too hard on my comparisons, i shoulda took a freakin picture.

the check valve is a black hollow cylindrical rubber plug with a one way "duckbill" type valve on the end. the only comparison i can think of for the "duckbill" is the mouth on a whoopie cushin. a spring fits into a recess in the back of the plug which applies pressure so that it seals against the backside of the flared seat. there is a shoulder around the "duckbill", which is what seals against the backside of the seat.

when you step on the pedal the brake fluid goes through the one way duckbill valve. when you release the pedal the fluid will return, pushing the valve back into the port until the spring can overcome the fluid pressure. at that point the check valve seals against the backside of the flared seat. because of the design of the the "duckbill", fluid can only travel in one direction through it.

to check a master for these check valves all you have to do is insert a small drill bit or a jewelers screwdriver into the master port. if the bit bottoms out all the way into the port it doesnt have them. if it does have them the bit will barely go in the port and you will feel the rubber valve and a springing action.

to remove/install them you have to remove the brass flared seats in the master ports. to do this you will need a slide hammer and an appropriately sized sheetmetal screw. screw the sheetmetal screw into the flared seat opening and yank the seats out with the slide hammer. duburr the hole in the flared seat and install the "duckbill" end into the back of the seat, then press the seat back into the master port. an easy way to do this is lightly tap the seat in the port and then screw the brake line/fitting in and tighten it down. try not to damage the seat area otherwise it will leak.

hopfully that makes sense.

Thanks for the helpful description! After checking, it seems that I have no residual check valves in the old or new MC.

In my other post, I'm asking about the proportioning valve. Maybe it has the residual check valve built in??

i seriously doubt that the prop valve on your car has any check valves in it. my 67 biscayne as well as a 68 firebird project i have do not have any check valves in the block(ive checked).

as a matter of fact, the master i robbed the check valves from was the original unit off of that 68 firebird.

unfortunately i couldnt find any sources for the original type residual check valves at all except out of an original master cyl. if you cant find a master with them, you will probably end up having to add an external residual valve.

You are probably right. I see Wilwood makes the residual valves, in 2 and 10 lb units. An earlier post mentions using a 6 lb unit. I wonder if the 10 lb Wilwood would create too much pressure at the rear brakes?

you wont have any problems with the 10lb unit, ive seen numerous people on this site with them and they've had no problems. besides, no matter how much residual pressure you have you can adjust the rear drums however tight or loose you want to make up for the difference.

Originally posted by northern 396:
My earlier post outlined the lack of brakes in the rear and the pedal that nearly goes to the floor. (1967 Chevelle, Disc front, Drum rear, power brakes.)One of the biggest factors contributing to low pedal with drum brakes is oversize drums. No matter how much you adjust the shoes, no matter with/without residual valve, there will be slack between [wheel cyl/push pins/brake shoe]. Only way to get rid of the slack is thicker linings on the shoes or new drums. Measure your drums. If they're more than .030" over, replace them before you do anything else.

The m/c rebuild kits pictured by Bendix show residual valves with some kits. Not to say they are actually in the kits, but they are in the pictures.
http://img227.exs.cx/img227/831/95d45em.jpg


I think some of the service replacement GM combi. valves come with residual valves in them...