master cylinder or booster problem?

[1975DusterGuy]

I have an issue with my brakes: my brakes seem to work as expected when I'm slowing down, under light braking. But as I get closer to that stop sign/light and begin pushing hard to stop, they become pretty hard, almost like there's no pressure at all, which can be a little scary at an intersection. Here's where it gets tricky...

I noticed the bottom of the master cylinder covered in brake fluid like it was leaking. I have the original 1975 mechanic's manual for my Plymouth Duster (stock 318 with a new Edelbrock 4bbl and performer intake) which gives me the procedures to check the brake system. I went ahead and did that just to make sure and it seemed to point to a MC on the fritz which confirmed my suspicion. In doing some additional research through the brake threads on here, it seemed like a lot of guys were saying hard pedal is probably an issue with the booster. The MC is remanufactured I put on probably 5+ years ago. The booster is original to my knowledge.

My questions:
1) Does this sound like a MC or booster issue? Or both?
2) Stock bore size on my MC is .937" according to the manual. MPBrakes has a 1" bore MC. Will this work with the stock push rod?

Thanks in advance!
Ben

 

[805moparkid]

with the engine off pump the brakes till it gets hard, this will get rid of vacuum assist.

then press on the pedal almost as hard as you can and hold. if it starts to do down or get soft then the cups in the master are leaking, or the wheel cylinders.

this is just a test for more info

 

[TrailBeast]

That harder feeling is when the brake piston bottoms out on the spring for the dual tank piston.
This means the front brakes are fading because of the leak that is causing fluid on the cyclinder and it is relying on the rear brakes only.
Betcha if you pushed the brakes real hard it would skid the rear tires only.

It's the master cylinder.

Very rarely a booster can get a crack in the rubber diaphram that only leaks if the diaphram moves enough to open the crack.
So it is possible, but you need a master cylinder anyway, so start there and see if that fixed it.

805,
There are two cup seals in there so one can be bad and the other can hold.
When this is the case, holding the brakes does not show the leak.
He has fluid on the bottom of the master, so that's a pretty good clue if it's not the lines or cover leaking. (usually a rear piston cup leak)

 

[805moparkid]

That harder feeling is when the brake piston bottoms out on the spring for the dual tank piston.
This means the front brakes are fading because of the leak that is causing fluid on the cyclinder and it is relying on the rear brakes only.
Betcha if you pushed the brakes real hard it would skid the rear tires only.

It's the master cylinder.

Very rarely a booster can get a crack in the rubber diaphram that only leaks if the diaphram moves enough to open the crack.
So it is possible, but you need a master cylinder anyway, so start there and see if that fixed it.

805,
There are two cup seals in there so one can be bad and the other can hold.
When this is the case, holding the brakes does not show the leak.
He has fluid on the bottom of the master, so that's a pretty good clue if it's not the lines or cover leaking. (usually a rear piston cup leak)

lol you got me wondering if thats whats wrong with my swinger lol...

 

[1975DusterGuy]

Thanks for the info, guys! Guess I'll start looking for a good master cylinder and cross my fingers that's the issue. Luckily that's only a $60 part compared to a $400 booster/MC setup...