Mater cylinder help

[reborn55]

Master cylinder on wife's coupe needs to be replaced--small leak. Thought it was from 76 Nova---car has later model Nova clip--calipers, rotors etc. So I ordered brand new one for a 76 Nova---looked identical, painted it all up, bench bled it, installed it and no fluid tot he rear brakes. Tried to re bleed it at master cylinder, very little to no fluid came out of the rear port. Bad master cylinder or wrong valving inside of the unit. The one I ordered fits from 71-77 Nova, Chevelle etc. Casting number-6470409 come back as 70 Chevelle but none of the ones I have found look the same. I thought most setups would work--old cylinder works great--just a little weepage at the rear of it and around the gasket. Any help would be appreciated..Thanks

[chevynut]

Is the pushrod retracting far enough? Maybe it's too long.

[reborn55]

it is the new one not working and it looks identical

[Rick_L]

How about the adjustment of the pushrod?

Also did you "bench bleed the new master cylinder?

[reborn55]

I did bench bleed and it is basically a stock matched system from a 76 Nova I think. Am getting another new cylinder tomorrow and give it a try

[reborn55]

well got another new master cylinder today and the same problem--upon a little close inspection I found that there is a stop screw in the front reservoir that is on the coupe and the new one doesn't have it. Push rod stroke is also different. So tomorrow I am going to try a master cylinder that has the Delco casting number(which I have evidently) and see what happens. Kind of hit or miss when you don't buld it yourself--thanks for help

[Rick_L]

I really doubt that the "stop screw" is your issue.

You never did respond about the pushrod. Actually it's pushrods. The one between the booster and pedal should have no preload; the clevis pin in the brake pedal arm should rattle. I.e. no preload, but also very little play. You can adjust that one. Also the pushrod between the booster and master cylinder should have no preload either; it should have a small clearance, .015"-.030". Some of those are adjustable and some are not. If you have preload, the master cylinder piston won't retract enough to get a fresh charge of fluid when you take your foot off the brake. This will prevent being able to bleed the brakes and will cause "dry" brake lines. It's a bigger problem on the rear than the front.

Do you have a factory type combination valve? It has a shuttle valve in it that will shut off fluid to one end of the system if that system has a leak. This could occur during bleeding even. The cure is to temporarily create a leak in the front by opening a bleeder, and pushing on the pedal slowly but with moderate force. If the shuttle moves you'll feel it.

Last thing is that you could have a swollen shut brake hose at the rear. Often when the hose fails the rubber expands, closing off the hose. In fact, I would start there, disconnect the rear brake hose at it's front fitting and see if you get fluid.

[reborn55]

OK--all good suggestions and I would probably have done all of them and have with aftermarket applications. The Brake unit--power booster is from a donor car--matched GM unit. Delco numbers on both pieces--so why if I replace with supposedly the exact unit is there any problem with the brake rod adjustment. My biggest problem I believe is the depth(pedal travel) between the 2 cylinders. When bleeding the brakes with the old cylinder the pedal goes all the way to the floor when I open the bleeder valve and with the new one it only goes down about 3/4 of the way. They do make 2 different master cylinders for application--one with the Delco castings and the other which is for a Bendix version I guess..going to get the Delco stamping number tomorrow. If I knew the kits were the I would just go ahead and rebuild the one I have. All hose to the rear are new when I changed rear end. old cylinder works and brakes are great--just a little weepage at the rear of the cylinder. thanks for help---factory combination valve. Also in looking at factory Chevelle manual they show a little difference in the Delco and Bendix with the Delco having the screw stop in it

[chevynut]

Until today, I don't see where you said you had a brake booster. I still think this is a pushrod issue in any case. One way or another the master cylinder piston isn't coming all the way back. If you get fluid out of the rear port of the MC on the bench, but nothing comes out on the car, the booster pushrod is probably too long for the MC you're using. That would also explain why your pedal doesn't go all the way down...the piston is bottoming. Your new MC is probably not "exactly" the same as the old one.

Measure the length of the pushrod protruding out of the booster and then measure the depth of the hole in the MC from the interface with the booster.

[reborn55]

All fixed--Bought a Delco master cylinder--bench bled--installed screw stop and installed---brakes great no leaks and happy wife--she is driving 150 miles to a show this weekend.--thanks all---appreciate it--Guess Delco and Bendix don't interchange.