DEATH of a Master Cylinder????

[mbryson]

In my seemingly never ending quest to get decent brakes on the YJ, I ran another quart of fluid through the system today trying to bleed all the air out. It was a little cold to just 'gravity bleed', so by the time I got to the front, I was getting a little impatient (brake fluid resembles 90 weight) and started using the pedal to pursuade things a little.

As I started using the pedal, I started getting HUGE bubbles (2-3" long in the little bleeder tube) on the caliper nearest the master cylinder (drivers front). They would 'bleed' out via gravity, but every time I hit the pedal I'd get more air.

Do I have a bad master cylinder?

 

[wr250]

look under the m/c for wetness. if its sucking air, its pushing fluid out as well (and if it isnt it will soon). the bleeder screws can draw in air around the thread when you let off the pedal as well. options:
1. get a russell speed bleeder (has a one-way valve to prevent air from coming back through the bleeder, and fits tighter than the stock bleeder to stop air suckage around the threads)
2. get a brake bleeding kit and use it
3. get someone to pump brakes and bleed conventionally

 

[mbryson]

look under the m/c for wetness. if its sucking air, its pushing fluid out as well (and if it isnt it will soon). the bleeder screws can draw in air around the thread when you let off the pedal as well. options:
1. get a russell speed bleeder (has a one-way valve to prevent air from coming back through the bleeder, and fits tighter than the stock bleeder to stop air suckage around the threads)
2. get a brake bleeding kit and use it
3. get someone to pump brakes and bleed conventionally

I did notice some fluid underneath the master cylinder. I cleaned everything off with brake clean and will try again tomorrow as I think I 'overfilled' the front res. once causing the 'spill/leak'.

 

[Todd Adams]

In my seemingly never ending quest to get decent brakes on the YJ, I ran another quart of fluid through the system today trying to bleed all the air out. It was a little cold to just 'gravity bleed', so by the time I got to the front, I was getting a little impatient (brake fluid resembles 90 weight) and started using the pedal to pursuade things a little.

As I started using the pedal, I started getting HUGE bubbles (2-3" long in the little bleeder tube) on the caliper nearest the master cylinder (drivers front). They would 'bleed' out via gravity, but every time I hit the pedal I'd get more air.

Do I have a bad master cylinder?

Not necessarily. Here is the problem with bleeding brakes by pumping them. You can draw air into the master cylinder, let me explain. Master cylinders have cup seals, two of then actually, one for the front brakes and one for the rear. The front brakes have the large rear (towards the firewall) reservoir and so the rear of this cup seal is exposed to air. When you step on the brake pedal, pressure forces the cup against the side of the bore. When you let off, air can go around the lip. Any time you use this technique to bleed brakes you want to let the pressure off the pedal very slowly. You compounded the problem while in the cold since the rubber lip seal is not as pliable under normal temperatures.
For this reason it is ether best to use a pressure bleeder or a vacuum bleeder to bleed brakes. It is also a good idea to do it while the vehicle is warm.
There are some other things you can do to improve the stopping power of a lifted YJ or for that matter any vehicle that you have put larger tires on. First install soft linings instead of the harder linings that last longer. The friction coefficient is lower and takes less pedal force to stop. This applies to the rear shoes as well if you have not upgraded them to larger drums or discs.
There are double vacuum chamber boosters as well as hydro boosters available for the YJ. I see them advertised in JP magazine all the time so your problem is not unique by any means.
My first solution was to install larger drums on the d35 housing which helped a lot. (I think I still have those kicking around if you are interested Marc) Then when I had the 44 built I went with discs on the rear.
Todd

 

[TimB]

I have a vacuum bleeder pump - it works great. I'd reccomend getting one to bleed brakes and clutch systems.

 

[mbryson]

Not necessarily. Here is the problem with bleeding brakes by pumping them. You can draw air into the master cylinder, let me explain. Master cylinders have cup seals, two of then actually, one for the front brakes and one for the rear. The front brakes have the large rear (towards the firewall) reservoir and so the rear of this cup seal is exposed to air. When you step on the brake pedal, pressure forces the cup against the side of the bore. When you let off, air can go around the lip. Any time you use this technique to bleed brakes you want to let the pressure off the pedal very slowly. You compounded the problem while in the cold since the rubber lip seal is not as pliable under normal temperatures.

Good to know.

For this reason it is ether best to use a pressure bleeder or a vacuum bleeder to bleed brakes. It is also a good idea to do it while the vehicle is warm.
There are some other things you can do to improve the stopping power of a lifted YJ or for that matter any vehicle that you have put larger tires on. First install soft linings instead of the harder linings that last longer. The friction coefficient is lower and takes less pedal force to stop. This applies to the rear shoes as well if you have not upgraded them to larger drums or discs.
There are double vacuum chamber boosters as well as hydro boosters available for the YJ. I see them advertised in JP magazine all the time so your problem is not unique by any means.

I have a vacuum bleeder pump - it works great. I'd reccomend getting one to bleed brakes and clutch systems.

I'll have to try a vacuum bleeder and see if that gets me anywhere. Sounds like it might be worth my time. Thanks for the advice!

......My first solution was to install larger drums on the d35 housing which helped a lot. (I think I still have those kicking around if you are interested Marc) Then when I had the 44 built I went with discs on the rear.
Todd

I've got a 14 bolt in the rear with the stock Chev drums (for now) and a D60 front with the stock GM calipers. I am a little worried about the volume the stock Chev calipers require and if the YJ master can keep up (seems to work OK for Braden on his Ford D60).

 

[kirk86CJ]

I got a used MC from a mid 80's suburban for $35.00. I then got an adjustable proportioning valve from Summit ($80.00) - it reduces the pressure to the front, then sends it to the rear.
The reason I did this was I couldn't keep the rear axle from "pushing" me after I got the 4;88's, when not in 4 wheel drive. (I about drove off the front of the trailer - I had to slip it in nuetral)

Now, my brakes work fine - d30 discs, d44 drums.

 

[mbryson]

I got a used MC from a mid 80's suburban for $35.00. I then got an adjustable proportioning valve from Summit ($80.00) - it reduces the pressure to the front, then sends it to the rear.
The reason I did this was I couldn't keep the rear axle from "pushing" me after I got the 4;88's, when not in 4 wheel drive. (I about drove off the front of the trailer - I had to slip it in nuetral)

Now, my brakes work fine - d30 discs, d44 drums.

Is that the same booster as the YJ?

 

[kirk86CJ]

I think they are all kinda the same thing. The drum (or whatever they are called) was 1" too big in diameter to not hit the hood. I had to lower it by 1" when I attached it to the firewall. Then, the rod going through the firewall was 1' lower than it should have been to attach to the brake pedal. I just cut the tab off the rod, and welded a spacer on it to make an offset - getting my 1" back. I would just find one with a little smaller drum, so you can attach it at the right height - or do what I did. It was pretty easy.

One note of caution if you go this route. The tab on the end of the pushrod was threaded on the push rod with about 1" of threads. I unscrewed it about 1/4" to get the brake pedal in the right spot. I then test drove it, and pushed on the brake pedal real hard to test. When I did this, the rod broke in half right at the threads, which made my brake pedal go to the floor. I luckily was able to use my emergency brake to get back home. After this, I used a piece of Sch 40 pipe to "sleeve" the rod, once I got it the right length. Its kinda red neck, but it works.

 

[greenjeep]

I got a used MC from a mid 80's suburban for $35.00. I then got an adjustable proportioning valve from Summit ($80.00) - it reduces the pressure to the front, then sends it to the rear.
The reason I did this was I couldn't keep the rear axle from "pushing" me after I got the 4;88's, when not in 4 wheel drive. (I about drove off the front of the trailer - I had to slip it in nuetral)

I had a proportioning valve on my CJ (Chevy 3/4 ton booster and m/c with stock brakes) and I never touched it, and finally removed it. How can you tell which axle is braking and which isn't? I always figured it was better to have more braking power in the front where most of the weight and momentum are. With my 4.56s and 37 I have a very hard time stopping in 4-lo, especially going down hill.

 

[greenjeep]

First install soft linings instead of the harder linings that last longer. The friction coefficient is lower and takes less pedal force to stop.

This may be a stupid question, but what does the above mean? Are you talking about pads, rotors, what?

Thanks.

 

[mbryson]

This may be a stupid question, but what does the above mean? Are you talking about pads, rotors, what?

Thanks.

I think he's talking about pads. It made a HUGE difference with my marginal XJ brakes.

 

[ace]

Did you bench bleed the master before installing it? You accomplish this basically by feeding the outputs back into the resevoir through plastic tubing and pumping it until the bubbles are out before attaching the brake lines and pushing all that air through the system. It took alot of pumping to purge the last one I put in.

 

[mbryson]

Did you bench bleed the master before installing it? You accomplish this basically by feeding the outputs back into the resevoir through plastic tubing and pumping it until the bubbles are out before attaching the brake lines and pushing all that air through the system. It took alot of pumping to purge the last one I put in.

I replaced the MC with an '86 E350 unit ($19.99 at Autozone). Bench bled, and bled at the calipers.....works AWESOME