Master cyl problems!

[Longknife]

When I purchased my '69 RR in 2019 it had been converted to (non power)disc brakes with the adjustable valve to the rear and a new master cyl. The brakes worked great and stopped the car very good. After a summer of driving, the brake pedal seemed to be getting low. It was solid but would travel about 1/2 way to floor before pressure was felt and then it would stop great.. A single pump would bring the peddle back up to the top. I tried adjusting the rear brakes but it made no difference. I put the car away for the winter and next spring I checked the brake adjustment and even bled the brakes but a test drive confirmed they peddle was still going too far down and the more I used the car the farther the peddle went down. A single pump would bring the peddle back up. I purchased a new MC, spring of '21 and installed it and that brought the peddle back up where it needs to be and stopped the car nicely. Now that cyl is doing the same thing. There is no more than 5,000 miles on this car and I have gone through two master cyls...???? This is the one I purchased last spring,,,What gives?
https://piratejack.net/1960-1970-mopar-master-cylinder/

 

[hangn0ut]

Fluid level? Pad wear?

 

[1STMP]

Hello LongKinfe...
I was having the same difficulty and at my
wits end. Though I have a dual diaphragm
booster my brake symptoms were the same.
Using the following link I followed their advice
to track down the problem.
https://www.getdiscbrakes.com/knowledge-base/tech-articles/causes-of-low-brake-pedal
The problem turned out to be the bleeders
in the front calipers not being at their highest
point. I removed both calipers and used a piece
of wood the same thickness as the rotors to be
able to rotate the bleeders. (you'll need a helper
to pump the pedal). It worked as I was able
to get all of the air out of the system.
Hope this helps.

 

[Longknife]

I will check my calipers today and see how high the bleeders are. Thanks, LK

 

[Longknife]

I checked my calipers; The bleed screw is tight on top. no problem there. I think The Master cyl is bad again. Can anyone recommend a decent one? I would like one to look as close as possible to OEM equipment. I don't want an aluminum or plastic one.... Thanks, Ed

 

[Rusty 72]

Something in that set up is wearing out that master cylinder. Mismatched parts maybe?

 

[1STMP]

I checked my calipers; The bleed screw is tight on top. no problem there. I think The Master cyl is bad again. Can anyone recommend a decent one? I would like one to look as close as possible to OEM equipment. I don't want an aluminum or plastic one.... Thanks, Ed

It's not whether they're tight or not. They need
(the bleeders) to be rotated off the rotor and
tilted up to the highest point. I used a piece of
wood the same thickness as the rotor to stand them up, and had someone help with pushing
the pedal.

 

[Longknife]

That should read RIGHT on top. not TIGHT on top....

 

[Longknife]

Something in that set up is wearing out that master cylinder. Mismatched parts maybe?

I don't know what would be mismatched, it was converted by the guy that restored the car, it appears he used a new conversion kit as all the parts are brand new. It has the stock looking calipers rotors etc in the front and factory drum brakes in the rear. It also has the adjustable valve going to the rear. I have never messed with the valve because the brakes work great and stop the car nicely. They are not power, but they stop the car nicely with minimum effort. But as I stated before, the more I drive the lower the pedal gets. I've gone through two master cyls in two years. About 2500 miles each. I'm about to pull my hair out and I'm already bald!!!
jmn

 

[MIKESPOLARA]

I checked my calipers; The bleed screw is tight on top. no problem there. I think The Master cyl is bad again. Can anyone recommend a decent one? I would like one to look as close as possible to OEM equipment. I don't want an aluminum or plastic one.... Thanks, Ed

I think you just have had 2 go bad, who knows how old the first one was and the second, well you bought from MBM (Pirate Jack) cheapest China stuff they can get.
Try this raybestos from rock auto about $46
RAYBESTOS MC36283

 

[Ranger16]

I think The Master cyl is bad again. Can anyone recommend a decent one?

I replaced mine (manual brakes) with one from O'Reilly's. Looked like the original except for the cover. Swapping covers was simple.

 

[Longknife]

I think you just have had 2 go bad, who knows how old the first one was and the second, well you bought from MBM (Pirate Jack) cheapest China stuff they can get.
Try this raybestos from rock auto about $46
RAYBESTOS MC36283

Well, I failed to mention the first one was new too. It was a bare casting and it had already began to rust. The one from Jacks was powder coated black cyl so I ordered it. Most of the classic car part dealers sell this same m/cyl. with huge price differences. Yeah, I know about' cheep China Chit! I ordered some Brand name brake hoses for another job because I wanted the best. They were twice the cost of the Chinese ones. When I got them the Box had the brand on the outside but the hoses were marked "CHINA", I think I am going to orderer a rebuild kit for my master cyl. made in America....LOL!!!!