weird brake problem. . .help me

mcurcio1989 · 10-15-2009, 03:33 PM

So two days ago i put switched from the manual brakes that came stock on my 76 cj7 to a 1995 yj master cylinder and booster. I am running the same combination valve and all the lines.

It was working great loads of stopping power with way less effort than before. I drove it for maybe 30 city miles and it worked perfectly. So yesterday i got in the jeep and started to reverse, the brakes were a little bit stuck with pressure on them. Whatever it went away after a second and was back to normal. They worked fine for a half mile (i hit them a couple times). Then i hit them and it was like half the pressure was there and when it is fully depressed (about 2/3 of the way to the floor) it is giving me maybe 60% of the deceleration as before. not sure if the random pressure has anything to do with this.

I am totally confused. I checked all the wheel cylinders and none seemed to have blown out or gone bad. Also the fluid levels did not drop. I have no clue why it would just all of the sudden do this. Its like the first half of the range feels normal but then after that it doesn't increase at all. I would greatly appreciate the help here.

jeeperman19 · 10-15-2009, 03:57 PM

I am interested..... I was planning to do that swap on mine, Good luck

**Cutlass327** · 10-15-2009, 06:08 PM

Try re-bleeding them? Could be the MC is now bleeding past internally, how long was the MC sitting before you used it?

ksheffer08 · 10-15-2009, 09:34 PM

is the master cylinder new or used? did you bench bleed it?

i would try bleeding brakes again, and make sure your vacuum lines are tight. otherwise, you may have a bad master cylinder. <- this is a common problem with re-man'd MCs

mcurcio1989 · 10-16-2009, 06:04 AM

Well i have bled it a couple times now and there is no air coming out at all. The master cylinder is used and i got it off ebay so it could have been sitting for a while. It did not look to bad tho. Vac lines are tight i played around with that for a while.

How do i bench bleed the master cylinder?

Do you think getting a rebuild kit and honing the mc would solve this if that is what it is?

What is so confusing for me is that it worked great and then all of the sudden went to this. My understanding is that m/c go bad overtime not just all of the sudden. If it had started out like this i would have assumed the m/c. What is your take on that? Thanks for the help guys i really need to get this taken care off!

ksheffer08 · 10-16-2009, 06:13 AM

if you google bench bleed there are right ups everywhere...

its possible there is a vacuum leak in the booster. or an internal MC leak. I wouldn't recommend buying used brake parts ever. I don't even like reman'd MCs. so you didn't touch anything beyond the booster and master cylinder?

if you press the brake pedal down and hold it... does it slowly (really slowly) keep going to the floor?

**bigredisb** · 10-16-2009, 06:13 AM

[quote=mcurcio1989;8114081]
What is so confusing for me is that it worked great and then all of the sudden went to this. My understanding is that m/c go bad overtime not just all of the sudden.QUOTE]

I have had a couple of MC's go bad and recently one on my CJ (not power brakes) and all have gone good from one minute to completly toast in another minute. MC's are pretty cheap. I bought mine for manual brakes for 18 bucks remaned from the local jip and run parts store. I took my core apart and the only thing I could find was a couple of tiny (could barely see with the naked eye) pieces of contamination. I would imagine your MC being used was probably shipped with no plugs in it so it could have been a small piece of trash that is causing this.

**Cutlass327** · 10-16-2009, 06:49 AM

Would probably be cheaper to get a reman one than the kit to rebuild it yourself..

smoothop · 10-22-2009, 12:08 AM

I had the same problem on my jeep (granted I am running a diesel with a vacuum pump with the stock CJ7 Braking system)....at Idle I would have horrible braking power, but when the engine had load it would brake decently, I got sick of the finnicky hit or miss vacuum boosters/mc's...Thank God for hydroboost.

mcurcio1989 · 10-22-2009, 10:46 AM

hm I'll bet it is a small piece of contamination in there. That seems like it would make sense to explain the good one second bad the nest deal.

Looks like my local autozone has one in stock for 44. One of my friends has a 88yj with the same mc. I am kind of debating trying his on mine to see if that is the problem. How does the mc disconnect from the booster. Ie is their a single linkage running from the mc to the pedal or is there a joint so i could disconnect the mc from the booster.

btw the booster works great. Its just somewhere along the line i am not getting the proper pressure.

mcurcio1989 · 10-23-2009, 12:30 PM

okay so a replaced the master cylinder put new fluid in and bled them very thoroughly . . .basically its no different. So that was kind of a waste. I have no clue what it could be at this point. The odd thing is that it almost seems to be sporatic. Sometimes i can get some pretty good braking power and others its like half of what it should be. . . do combination valves go bad?

If my wheel cylinders are bad i would notice the fluid going down and some fluid inside the drums right? I am not seeing that.

I am kind of wondering if the front passenger side wheel cylinder is bad. The front driver side was replaced so its good. One time it did seem like the front fluid level was low but i could not remember if i had topped it off after bleeding.

Usually the power is decent when i start out and gets worse the longer i drive. That seems like it may point to fluid leaking onto the drum. . .

Mike Romain · 10-23-2009, 12:52 PM

A bad front wheel bearing can cause strange pedal pressures which can shut off the combination valve. When you corner the disk pushes the pads and piston back into the caliper, then when you hit the pedal, it drops low to fill it back up and the combo valve can see this as a leak which shuts off two wheels from braking.

I would be jacking it up and shaking the tires to see if one side is loose.

**rixcj** · 10-23-2009, 12:54 PM

When you bled the brakes, did you have the button on the proportioning valve pressed in?

Rich

mcurcio1989 · 10-23-2009, 12:55 PM

^This is a 76 so it has drums all around. I am guessing no but does what you were saying apply to drums?

Mike Romain · 10-23-2009, 01:04 PM

Quote:

Originally Posted by mcurcio1989

^This is a 76 so it has drums all around. I am guessing no but does what you were saying apply to drums?

I really don't know, but it may. Thinking the shoe can be pushed in, but the other side may go out at the same time so I am not sure.

It would only take a minute to check the wheels for slop though.