Brake Woes

My CJ is a 1970, with the stock dual master cylinder with bolt on cap. I converted to 11" drums all around, but hydraulically speaking the system is all original. When I hit the brakes I can come to a stop, but if I continue to hold pedal pressure, say at a stop light, the pedal will slowly drop down almost to the floor. I can pump it up, but it'll drop again. No sign of fluid leaks that I can see. I'm thinking the master is shot, am I missing something?

 

yep if no wheel cylinders leaking
replace MC

 

exactly X2, plus do not delay, if the master is leaking internally it can and will fail completely and suddenly when you need it the most. Guess how I know!

 

Use that 6.32 first instead of the brakes

 

Symptoms of the master cylinder but it looks like your overdue on stoplight switch replacement.

 

Nah, this one is good for another month or two

 

So, whats a good source for this? Looking online and I keep getting results like these...neither is correct.


Edit: Jeepsterman has the right one but it's kinda spendy.

 

Remember to drill and tap the front/rear of the reservoir for remote fill not the sides...

Crown Automotive Brake Master Cylinder Brake https://a.co/d/a9SpgyM

 

1.Don't trust pictures to be a picture of the item you need. Even if the picture states otherwise. 2.Although hard to do, I would verify the part is NOT made in China, my experience says quality from China is spotty at best. 3.Buy from a reputable store or site recommended by people here, with recent, good experience buying the same product you are looking for. Good luck!

 

just did this not too long ago. oreillys in my area is getting out of the reman master cylinder business.
NMC11454 is for a new master cylinder for 30 bucks more than the reman one you listed and comes with a lifetime warranty

 

The cap is the issue on the NAPA unit? Been a long time since switched to swinging pedals. I forget if the old frame mounted unit I had was the bolt on cap or the flip over spring clip thingy.

Maybe it's a file photo. I'd probably have them get one in and see if it's like the one the picture or if it's correct.

 

Fwiw This is the one currently on my rig, pulled from my shopping history. It was really clean and flashed rust pretty quick.

20221230_131447 by Joe with a jeep, on Flickr

 

The bolt on cap is stock, like the one in Joe's pic.

 

You might want to pull the MC and disassemble it to take a look. If the cups are serviceable you may be able to hone it.

While not a permanent fix, this may get you to your next paycheck.

When mine went bad many years ago, I swapped a 72-75 MC in. I had to install adapters for the lines but it worked. At the time the replacement MC was about 120 bucks and the later model around 40, so I went that route.

 

Well, I ordered a replacement from Rock Auto. The picture showed a bolt on cap, as in the pictures above. What arrived was one with a bail holding the cap on.
But then I got to thinking, why not use this style? It'd certainly make it easier to check the fluid. It should be sealed up better (no vent in the cap). Assuming the fittings are correct sized, I think I'm going to use this one.

 

Put this on my 1971 a couple months ago. Fit perfect

 

Does that have master cylinder have residual pressure valves built into it like the original? If not, you should add some inline. 2 lb for disks, 10 lb for drums. The valves keep the disks from retracting back too far, and they keep a little pressure on the lip seals in the drum wheel cylinders so they don't leak.

The original master cylinder has the residual valves incorporated under the nuts on the exits:

 

The one in my link has the stepped bore. The one you have may not. Take note of pedal feel and behavior. The stroke and rod length may also be different.

 

This is also a really good point. The factory master cylinder has a 7/8" bore for the front brakes and a 1" bore for the rear brakes. This increases the pressure to the front brakes for proper proportioning. Without a stepped master cylinder, you may need a proportioning valve in the rear brake line to prevent premature rear lockup.