cj7 master cylinder

[juggernaught]

I am aware that the vacuum booster is for a low rpm boost to the brakes. But, if air IS in the line I need to remove that first.

I am not discounting the propotioning valve....my question was "Do you need to open the p-valve while performing a power bled? I would like to know if it is absolutely necessary. My limited understanding of the p-valve is that it is always "open". Am I wrong?

[Mudrat]

But, if air IS in the line I need to remove that first.

I am not discounting the propotioning valve....my question was "Do you need to open the p-valve while performing a power bled? I would like to know if it is absolutely necessary. My limited understanding of the p-valve is that it is always "open". Am I wrong?

I never have on ANY of my trucks, from the M38A1, Land Cruiser to Chevy K15, to any of my Jeep Trucks (CJ, YJ, SJ's). Bleed through it. Start with the furthest wheel (right rear) and work up.

[Old Rugged Crosser]

But, if air IS in the line I need to remove that first.

I am not discounting the propotioning valve....my question was "Do you need to open the p-valve while performing a power bled? I would like to know if it is absolutely necessary. My limited understanding of the p-valve is that it is always "open". Am I wrong?

Here is why you should keep the P. valve open:

The P. valve restricts the pressure on the rear brake system. You do not want the front brake system to dictate how the pressure is applied to the rear. Same with the master cylinder,
if air is trapped in one segment of the plunger the part that has no air dictates how it operates.

To be successful in your bleeding process you must first make sure there is no air in the master first, then on to the rear brakes with the p.pin held open.

[juggernaught]

Thanks guys.....we will see how it goes thursday with the power bleed.

[msalaba]

Disconnect the two brake lines that go into the master cylinder and fashion two more lines that will screw into the M/C and turn them up and into the two M/C reservoirs then pump the brakes slowly and do not go all the way down on the pedal. keep stroking like this until no more bubbles come. Then connect the original lines. You should see a lot of difference.

This is known as "bench bleeding" the master cylinder. There are generally 2 reasons to do it. 1) installing a new master. 2) original master was drained of fluid.

A power bleeder (basically a vacuum pump) applies a lot of vacuum to the system. With a good seal at the bleeders it will even pull air out of the master and assorted valving. Start at the furtherst bleeder from the master and work to the closest just like with manual bleeding.

I worked on a 79 CJ7 w/ manual brakes with the same problem. The brakes were really spongy and weak. It had 35" tires and all factory brake components. It turned out that the rubber lines had deteriorated over time and were ballooning. We went to S/S braided lines (mostly due to the custom length required for a lift) and braking performance was greatly increased. We also determined that a vacuum booster would be a waste of time. (Even had one in hand from a donor)

I feel that the hydro-boost system is grossly over-rated. They add complexity to the vehicle. They leak P/S fluid. And when you get one from the junk-yard they need a rebuild. They have a nitrogen charge that helps the unit maintain the manners of a vacuum booster. And you need a high volume P/S pump from a vehicle equipped with Hydro-boost.

If you are intent on throwing parts at it, get a factory vacuum boster and master for your application. The previously mentioned Corvette master could be used if you have 4-wheel disk brakes.

For 4 wheel disc, I have had good luck using Chev 1/2T masters (early 90's w/ plastic reservoir) and eliminating the Proportional and Residual valves from the system. It requires some modification to the brake pedal linkage so it is not 100% bolt-on. The lower rear PSI of the master combined with the removal of the proportioning valve = just about perfect front/rear bias.

I have a vacuum booster and master from a '83 Cadillac Coupe DeVille for 4-wheel disk brakes. (happened to have one in the weeds) I have a Dana 60 up front (factory brakes) and J20 calipers and rotors (Same as 70's 3/4T Chev D44 brake parts) fabbed to my D70 rear.

I could not ask for much better braking performance with 42" Irocks. I soiled myself the first time I locked them up. Most fun is watching your buddys over heating the steering and not have brakes! (Hydro-boost systems)

There is a lot of brake tech in Internetland but beware, there is also a lot of "tech guru's" offering BS. I can only offer advice on things I have learned trial-by-error.

my $0.02

[msalaba]

NPD was right. You may have to remove the calipers to get out all the air. It is a air problem that you are having.

Generally. With the use of a power bleeder this is not necessary.

I used quite a string of 4 letter words trying to bleed my rear brakes. A power bleeder solved the problem. 3 Years later I was servicing the rear brakes when I noticed I had the calipers on the wrong sides so the bleeders were not at the highest point. After switching the calipers around bleeding was much easier.

Why is it always the most stupid things that I overlook?

[juggernaught]

O.K..........so sometimes I'm a little slow. Got the brakes power bled. He said some air did come out. Brakes are firmer now with a much better feel. Then I realized that DUH! I don't have power brakes.

Never really gave it much thought as I have always had many other mechanical jeep projects going on. I think I always assumed that because I had disc calipers and power steering, that I had power brakes also.

I did come across a NAVAJO BRAKE setup that looks just like the ticket when I get ready to swap to the power set-up.

I did not need to hold the proportioning valve open to power bleed the brakes.

[Old Rugged Crosser]

As I have been reading the dialogue about brakes it reminded me to do what I have intended to do for a while and that is to change out my plastic reservoired m/c to the corvette m/c.

As I was bench-bleeding my new m/c using plastic see through hoses run into the reservoirs so I could observe the air bubbles escaping I observed some interesting phenomenon that would have gone unnoticed if the hoses had not been clear.

As I stroked the plunger I noticed that the front brake res. cleared of air very rapidly, pushing out air and pulling fluid back in. The rear brake res. took a lot longer to clear. I also noticed that the rear brake fluid took a noticeable delay before the fluid started moving in the hose. This, it seems, has a function of a hold off valve which delays the application of the rear brakes so that they don't cause the rear end to come around under hard braking.

I was not satisfied that all the air had been pumped out of the rear portion of the m/c as it stopped pumping any fluid through the clear hose when the front portion had been cleared of any air. I then decided to put my brake vacuum pump on the hose and sure enough there was air still in the rear plunger. Air that would not come out using the standard method. This would have caused me lots of trouble as was talked about above concerning Juggernaught's brake problems.

This tells me that using a brake vacuum pump is a very valuable tool when it comes to brake bleeding. Besides it makes bleeding the brakes a one man job.

[juggernaught]

Thanks..............will look into the vacuum pump. I talked to a mechanic that said they had a setup that used the working a/c unit as a vacuum to bled the brakes.