Power booster help?

Ok, so I'm working on this 88 MJ. I can't really get the brakes to lock up. The e-brakes are fine but I can't get any real pressure to the brakes when the engine is RUNNING. Engine off, pressure is good, but when it's on, pressure is gone. I pulled the Vacuum hose while it was running and the brakes get strong again. reconnect the vacuum hose and the pressure falls off again.

Any clues?
88 MJ 4.0 fuel injected, auto, 4x4.

Here's the deal: You appear to be confusing pedal feel with hydraulic pressure. Different animals. With the engine off, or with the vacuum line disconnected from the brake booster your pedal feels hard because you have no power assist; without the power assist, the actual hydraulic pressure being applied to the calipers/wheel cylinders is significantly less. What you want is a firm pedal with the engine running, and you don't want the pedal to get near the floor. If the pedal is soft, and travels too far down, this indicates air in the system. Air compresses, making the pedal feel "spongy", and results in a low pedal, and poor brakes. Hydraulic fluid cannot be compressed.
Another problem that can result in "spongy" brakes, are soft factory rubber flex lines at the front or rear brakes (two in the front and one at the rear).
Start by closely inspecting all brake components for wear and leaks. Any leak can result in air being sucked into the system. Next would be to properly bleed the brakes.

Now I get all that. I know there is a little bit of air in the line, but I tried the braking power with the vacuum line connected and disconnected. the braking power is definitely different when the line is disconnected. My brother and I rolled the truck down the driveway with the engine running. (we were safe about it) both with the vacuum line connected and disconnected. The vacuum line being disconnected resulted in much better braking power (ie. the truck stopped without the need of the E-brake)
With vacuum line connected, had to use the ebrake to get it to stop.

I understand hydraulics, I don't understand power boosters.

A unique thing to the MJ is the brake pressure/load switch located above the rear axle housing with a rod going to the axle housing. It limits brake pressure when the load is light and adds pressure when the load is heavier. Do you see it there?

cruiser54 said:

A unique thing to the MJ is the brake pressure/load switch located above the rear axle housing with a rod going to the axle housing. It limits brake pressure when the load is light and adds pressure when the load is heavier. Do you see it there?

Click to expand...

With or without the load sensing mechanism on the rear axle, the vehicle should have lighter break pedal feel and better stopping power when the engine is running and the vacumn booster connected than when the enginne is off or the vacumn booster disconnected. The discription of the pproblem given by Eric is totally opposite to conventional wisdom or am I not getting it?
With or without proper vacumn at the booster you should have some breaks. All the booster does is make the brake pedal easier to push when vacumn is present.

Despite my lack of understanding, I would suggest an inspection of the entire breaking system starting at the wheels working your way up to the master cylinder then bleed it correctly. I am not familiar with the load sensing mechanism on the MJ, but the early Toyota pickup that I DD have a similar setup and I love it. Works perfect,...well near perfect every time.

I agree techno1154, but we need to know if it's there sooner or later. The OP's description of the symptoms does seem opposite of conventional wisdom. Need to get the air out of the system first anyway.

Before performing the next tests, be sure to completely bleed the brakes.

To check your booster, turn off the engine and step on the brake pedal 5-6 times to empty the booster. Put your foot on the brake pedal with moderate pressure and start the engine. Your foot should fall slightly. If not the booster is bad.

IMO, it sounds like you MC may have a leak. If the bore in the master cylinder is pitted or the rubber seals have decreased in size then some fluid will bypass the seals under pressure giving a lower pressure to the wheels and a softer spongier brake pedal. The fact that the pedal is hard when the vacuum is removed makes me think that manual pressure is insufficient to cause the fluid to bypass the seal.

To test for this condition remove the brake lines from the master cylinder and plug the outlets (obtain outlet plugs from a local auto parts store). When you apply the brake pedal (vacuum connected, engine running) it should be high and firm. If it is spongy or slowly goes down then either the bore of the master cylinder is pitted or the rubber seals are bad.

That's why I'm so confused. The load sensor was removed before I got it and it had been T'd off. I removed the T and the return line I saw mentioned in other posts.
I am going to completely re-bleed the brakes again, but a little air in the line should NOT result in the symptoms I'm getting. The spongy-ness is there but it's not bad.

I'm pretty sure I'm going to have to test the Master cylinder before I go and waste another couple bottles of brake fluid. If that passes then I know the power booster and master cylinder are good and the problem is lower in the system.