So the other day I was on a ride with my local club. I dropped down, nose first, into a river crossing. It was very steep. Anyway, after coming out of the river, by brakes were not the same. The brake light would come on and I would have to pump up the brakes to get decent stopping power.

I looked in the reservoir and the liquid was a little low. The biggest thing that I noticed is that the rubber "boot" had not expanded to displace air in the cylinder. So, I think that I got air in the front lines.

I have tried and tried and tried to bleed the brakes since this event. I have never had so much trouble, even when the entire system was dry. I gravity drained some, followed by vacuum, followed by using the pedal. Well, I thought that I was done, but no.

Then I made a huge mistake. I had taken the cover off to watch the reservoir to see if air bubbles would come. I depressed the peddle slowly and released slowly while watching the fluid. I noticed that I could see the fluid moving in the rear reservoir, but could not see any movement in the front reservoir. Then I forgot that the cover was off, started the motor, and pressed the brakes. Very, Very, Very bad idea.

So I went back to bleeding. And just have not had any luck.

Where do you recommend that I start at this point? Has anyone used one of these power bleeders? Are they worth it and would they solve my problems?

Also, I looked at my combo valve and noticed that the pin was pushed all the way in the front. When I push the brake peddle the pin moves out (toward the front of the jeep). Is there anything that I need to do with this piece to get it to properly bleed?

Should I just let it set with the cover off (loosely on) to see if any air moves up the lines?

Please, any help would be greatly appreciated...

-- mike

Sounds like your problem is in the rear. When you compress air in the lines, it will expand back when you release the pressure and cause fluid to be pushed back into the reservoir. That should be what you saw.

The boot not expanding is not a problem, it just means there is a place in the seal where air can enter the reservoir rather than create vacuum. Applying the brakes with the engine running and the cover off the master is not a problem either. With air in the lines, it may have splashed some fluid out.

Gravity bleeding would work but if the air got in at the master, you can expect it to take a long time for it to exit at the bleed screw. Fill the reservoir, open the bleed screws and check the reservoir level every 10-15 minutes.

If you power bleed, DO NOT PUMP THE BRAKES UNTIL THE PEDAL BECOMES FIRM. By pumping, you will just scatter the air into a bunch of little bubbles. Open the bleed screw, have someone depress the brake pedal. Close the bleed screw and then have your helper release the pedal. Do this until you get air at the bleed screw or 5-10 times. Go the opposite side and repeat. Check for firm pedal. If the pedal goes to the floor or is real soft, repeat. If the pedal seems fairly firm, now you can pump before you open the bleed screw.

You should bleed the longest line first.

There are instructions to keep the plunger centered in the combination valve but I have never had a problem without blocking it.

i bet your master is junk. You probably sucked air and the pedal went down farther then it is used to going. Then with all the pumping and pumping down to the floor it made it worse.

Masters dont like to go to past there normal movements once they are in use.

As for the cap not pulling the rubber down. I cant get a master cover to seal up on my master. Its leaked from day one. I hate it. But im too lazy to change it. The fluid probably leaked out the cap during the trail ride and thats why it was low. I normally have to top of my brake fluid after a day on the trails. It just spills out.

If you want to try bleeding just the master, go pic up a master cylinder bleeding kit. Its basically two plastic fittings with some cheap clear hose that goes from the fittings to the tanks and it just pushes the air out and will suck fluid back in. Or if you want to get fancy, get a 12in brake line and make your own out of that. Just cut it in half and bend it around. You probably will need an adapter or two to do that though.

Thanks for the feedback guys. I wish that I had some good news, but that will have to wait until tomorrow.

Last night, after writing the post, I went back out to look at everything just to make sure I was not missing anything. I noticed that the front bowl was nice and clear and that I could see the bottom of the master. Well, the back bowl was very cloudy and I could not see anything. Well, it was full of entrained air. I ended up pumping the reservoir almost dry with my vacuum pump then refilled.

This evening I vacuum bleed all four lines. I ran a significant amount of fluid through each disk. The fronts had a lot of air. The rears, not so much.

Anyway, the peddle felt better after all the bleeding. But unfortunately, I could not start it. I had drained the battery from all the starts and stops so now it is on the charger.

If it does not work tomorrow, then I will replace the master with either an 1 1/8" or 1 1/4" bore -- either a 77 vette or a 73 Chevy C30. I read good things about both. I've also been eying a 9" dual diaphragm booster and a Jegs adjustable combo valve.

For the record, I'm currently running a stock power brake booster and master with 3/4 ton chevy brakes up front and Caddy Eldorado's in the rear.

-- mike

I used to think like this as well. I had a master cylinder that just wouldn't stay dry. When I posted my brake problems on here, Taz told me I had air in the system. Of course I argued that it was just crappy caps as my previous master cylinder leaked as well. But I've since reworked the booster/master cylinder connecting rod and I'm all sealed up. That cap that used to constantly spew, is dry as a bone. All day every day........... just my experience.

I would like to know where you can buy one of these kits. When I was struggling with my brakes, I wasn't buying a new master cylinder so I didn't have any of the "kits" that come with a new master cylinder. I went to a bunch of different parts stores, and nobody even understands what you are asking about. I'm amazed they could sell me a master cylinder without knowing what is involved in the installation.

The master cylinders come with plugs these days. No fittings and tubes to go back up to the top of the master cylinder. You are supposed to bench bleed these by pumping the master cylinder then cracking these plugs open to let the air out. I don't think these kits work as well as the older kits with the hoses. I'd much rather have some of the fittings with hoses that I could throw in my toolbox for that rare occasion that I have to tinker with my brakes.

I can't even BEGIN to count all the problems in this thread, but I'm going to address a few...

1. *IF*... You want MORE braking pressure at the wheel cylinders/calipers, you use a SMALLER BORE master cylinder, not a larger one.
Larger bore Master Cylinder will REDUCE your brake line pressure at the wheel pistons.

If you want more VOLUME, which you DO NOT NEED with 4 wheel discs, you use a LONGER STROKE, now a larger bore....

This is basic hydraulics.

2. I didn't see a thing about you taking the Proportioning valve apart and CLEANING it,
OR replacing it with something that is 'KNOWN GOOD'...

3. A Proportioning valve IS NOT NECESSARY with 4 wheel disc brakes!
The idea of a "Proportioning" valve is to delay application of the more powerful FRONT DISC brakes until the back brakes can apply, so you brake in a straight line,
Even when applying brakes hard in a curve.

Since all brakes will apply at the same rate with 4 wheel discs,
And since Jeeps normally don't have to apply hard braking at 100 MPH in a sharp curve, The "proportioning' valve is a hold over from more sophisticated braking systems, and isn't really necessary on all drum, or all disc systems except to turn the 'Brake Warning' light on.

4. Until you CENTER the spool valve in the "Proportioning" valve, you are NEVER going to chase the air out of your braking system... It's just NOT going to happen.

Any speck of rust WILL jam spool valve in the "Proportioning" valve CLOSED to one end or the other.
The tolerances in the "Proportioning" valve are VERY tight, and any little bit of anything will cause BIG PROBLEMS...

5. If you have trapped air in the system...
YOU MUST force the 'Foam' up or down through things.
It WILL NOT move or dissipate by it's self!

Most people think of trapped air as a BUBBLE, and it's not.
Trapped air is usually a FOAM, mixed with any moisture in the system (and there is ALWAYS moisture in the system!),
So it moves back and forth, compresses and decompresses with the master cylinder compression,
And generally makes a pain out of it's self!...

If you don't have the proper clip to keep the 'PIN' on the proportioning valve CENTERED while bleeding,
Try cracking a front brake line...

Open one rear caliper/bleed the rear,
Then go up front and take the pressure off the front side of the system.
You will usually force the foam out in just two or three sets of strokes like this...
And then you can go about bleeding like usual...

Sometimes leaving the system alone will reduce the foam to a single bubble or two, and that bubble will rise up and you can 'Burp' they system through the master cylinder, or push that bubble down and out a caliper/wheel cylinder.

Personally, I prefer to push it out because I don't like messing with brakes more than one day at a time!

I dont understand what adujusting the rod has to do with the master leaking out the cap. Mine leaks because the top of the master was never ground smooth just part way.

I havent bought that bleeder kit in a long long time. I have since made my own from steel brake line tube. I just keep a bunch of adapters around to fit what ever i need. The last kit i bought was at a federated auto, mom and pop, type auto store.

Quick update -- I finally got back out and played with the brakes again. Things appear to be improving. I just need my "helper" for an hour or so and I think that I can get it back to where it was. It is just too hard, and takes too long, to bleed the cylinders by myself.

Pump the brakes, use a block of wood to block the pedal at half stroke, open the bleeder, push the pedal full stroke and block again, being careful not to "release" the pedal, then going to close the bleed screw. Then doing it all over again..

Thanks for the response, while I agree with most of what you typed, this has me very confused. I would have thought that a smaller bore master cylinder would require more pedal pressure than a larger bore cylinder to achieve the same end of line pressure, assuming that the only difference in the system is the master cylinder bore size. I thought that the ratio of areas was inversely proportional to the ratio of pressures.

I'm not very worried about peddle pressure right now. I am more worried about stoke since I have "upgraded" both the front and rear stock CJ brakes to 3/4 ton brakes. I have not researched the hydraulic capacity of the new calipers vs the old calipers/drums, but I guess that I need to calculate whether or not I should increase the master bore size.

My "seat of the pants" testing, prior to my recent problem, indicated that I had more pedal travel with the 3/4 ton calipers than I did with the stock calipers/drums. This would indicate to me that I would need more area in the master cylinder bore to increase my pedal height back to where it was.

It is not a good day if I don't learn something new. Can you help me understand what I am missing with regards to the hydraulics?

My comment about replacing my stock combo valve with a Jegs adjustable is that I can retain the brake indicator light circuit, which is important to me, and so that I can balance my front to rear, if needed. It may not be needed. You are correct, since the Jeep primarily sees low speed conditions, the balance is not critical.

However, when I run it on the street, I don't want to lock up one end sooner than the other, if I can help it. I thought that brake lockup would be a fairly complex function of speed, weight distribution front to back both static and dynamic (standing still and while braking), the ratio of caliper size and ratio of pad area front to back, and disk diameter ratios.

Basically, if I am going to go through the trouble of replacing my brake lines, I want to make sure that I can adjust the system to account variables beyond my control without tearing the entire system apart again.

Thanks for all of the constructive help.

-- Mike

The fluid pressure can be measured in pound per square inch. To get the force, multiply the pressure by the surface area of the piston. As the piston area gets larger, the force gets larger.

A larger piston area at the brake will produce greater force for the same fluid pressure. A larger piston at the master will require more input force to produce the same fluid pressure.

That is about the only thing in TR’s post I agree with. You probably won’t be able to use a longer stroke master because you won’t have enough pedal travel. If the piston area of your upgrade brakes is too large for the master you could use a larger bore master but now you haven’t gained anything. You could use the longer stroke master by changing the connection point on the pedal arm but that will loose leverage and you haven’t gained anything.

About your only real option is to use the larger bore master with a power brake booster that will provide more assist force.

OK, I see where I screwed up my thinking now. Force = PressureXArea or pressure = Force/Area. In a closed system, pressure is constant therefor F1/A1 = F2/A2. Therefor F2 = A2/A1 * F1.

Because of this principle, the larger the bore of the master cylinder (A1) the smaller the Force applied to the caliper piston (F2), all other variables constant. That is what I get for mixing pounds force and pounds pressure...

Thanks for clearing that up for me.

-- mike