Soft Brake Pedal After New Parts And Bleeding

[Rymanrph]

I recently replaced the brake master and booster in my '97 swapped truck.  When the truck is off, the brake pedal is firm, but when the truck is running, there isn't much resistance.  It will stop, eventually.  I bench bled the master before installing it and I've bleed the entire system twice.  I double checked the rear drums to make sure they were adjusted properly and I can't find any signs of a leak in the system anywhere.

 

I tried to find someone near by that has a pressure bleeder, but haven't had any luck.  I need to get the truck inspected, but I'm not really comfortable driving it, so I'm kinda at a loss as to what to do at this point.

[HOrnbrod]

Try the booster test below. It might point you in the right direction. (Originally posted by Skidoo_i)

 

Test Power Brake Booster

If the pedal feels "hard" while the engine is running, the booster isn't operating correctly. If you suspect the booster is defective, do not attempt to disassemble or repair the power booster. Doing so is unsafe and will void your warranty.

Test 1

1. With the engine off, pump the brake pedal to remove any residual vacuum in the booster.
2. Hold pressure on the pedal while you start the engine. When the engine starts, the pedal should drop about a 1/4", this indicates that the booster is working properly.

Test 2  

1. Run the engine a couple of minutes.
2. Turn the engine off and press the pedal several times slowly. The first pump should be fairly low. The second and third should become slightly firmer. This indicates an airtight booster.

Test 3  

1. Start the engine and press the brake pedal, then stop the engine with the pedal still pressed. If the pedal does not drop after holding the pressure on the pedal for 30 seconds, the booster is airtight.

Inspect the Check Valve  

1. Disconnect the vacuum hose where it connects to the intake manifold. Do not disconnect the vacuum line from the booster. Air should not flow when pressure is applied, but should flow when suction is applied. If air flows in both directions or there is no air flow, the valve needs to be replaced.

Verify Enough Vacuum 

1. Check the operating vacuum pressure when the engine is at normal operating temperature. There should be a minimum of 18 in. of vacuum. Vacuum may be increased by properly tuning the engine, checking for vacuum leaks and blockages in vacuum lines

[Rymanrph]

I have the opposite problem; the brake pedal is soft while running, but firm when off.  I understand that it should be firmer when off, but it shouldn't be as soft as it is while running.

[HOrnbrod]

Assume your rear height sensing valve is long gone, si?

[Rymanrph]

Assume your rear height sensing valve is long gone, si?

 

Actually, no I still have it.

[HOrnbrod]

I've never had to do this personally, but others claim it works for a soft pedal cure if you still have the rear valve. Posted originally by Eagle:

 

Here's the official FSM procedure to follow when bleeding the brakes on an MJ that still has the two lines to the rear:

After bleeding the brake calipers and cylinders the rear brake by-pass line must also be bled.

Open a front caliper bleed fitting and depress the brake pedal to the floor. This will shuttle the by-pass differential valve and allow fluid to flow through the by-pass line. The brake warning light on the instrument panel will illuminate when the ignition key is in the ON position. This signals the shuttling of the valve.

Re-Bleed the rear brake cylinders with the front caliper bleed fitting open.

After re-bleeding the rear brake cylinders the entire system must be bled again.

Bleed the brake calipers and cylinders in the following sequence:

1st -- Right rear

2nd -- Left rear

3rd -- Right front

4th -- Left front

[Rymanrph]

That's certainly interesting.  I have speed bleeders on the front, so it won't be that big of a deal to do.  I probably should just get rid of that balance valve, but oh well.  I'll have to give it a try.  Thanks for the help.

[Rymanrph]

Well, I tried the above procedure for bleeding the height level valve and its probably a little worse than it was before. I have speed bleeders on the front that won't allow air back in, but when bleeding the rear with the front open, it should have the same effect; fluid can still flow out. I'm kinda stumped.

 

I drove it down the street and back and it will stop, but I just don't have much confidence in it. I'd hate to be in a panic stop situation.

[Eagle]

Well, I tried the above procedure for bleeding the height level valve and its probably a little worse than it was before. I have speed bleeders on the front that won't allow air back in, but when bleeding the rear with the front open, it should have the same effect; fluid can still flow out. I'm kinda stumped.

 

 

There's your problem.

 

In this procedure, opening the front bleeder is to simulate a front brake FAILURE. That means you have to allow fluid out and air in. You have to bleed the fronts again anyway in this procedure, so temporarily replace one side speed bleeder in front with a conventional bleed screw, bleed the two rear circuits, then replace the front speed bleeder and finish up.

[Rymanrph]

I used a regular bleeder again this morning and no change.   :fs1:

[HOrnbrod]

Sounds like it's not hydralics. Do you have another vacuum check valve you can pop in the booster?

[Eagle]

You are leaving the front bleeder open while doing the rear, correct?

 

Is the brake warning light on the dashboard lit when you open the front bleeder and step on the brakes?

[HOrnbrod]

After several brake upgrades with the original rear drums then with rear disks, retaining the rear load sensing valve in all cases, I never had to do the FSM bleed procedure for the rear prop valve line. Everyone it seems has plumbed out the original load sensing valve for various reasons; because it's junk, rusted out, supposedly is inop, or because they did not know how to adjust the lever arm w/o using the AMC 85* special tool for the valve arm vs. the valve housing markings. I suspect the latter, especially if any lift is involved.

 

IMHO, I don't think the OP has a hydraulic problem in his case, because I know he knows proper brake bleeding techniques.

[Rymanrph]

I don't get a brake light on the dash because I have a '97 cluster.  I tried swapping out the check valve from my old booster, but it behaves exactly the same.  I have never been impressed with the brakes on this truck, but I know that the pedal shouldn't go all the way to the floor (stopping point) with it as soft as it is.  I'm sure that my load sensing valve probably isn't adjusted correctly.  I may look in to eliminating it to see if that's causing the problem.

 

My registration/tax bill and inspection is due this month so I may just end up taking it somewhere.  I really don't like other people working on my vehicles, but I only have so much time and I'm really tired of bleeding the brakes by myself.

[Rymanrph]

I pulled this from another thread about removing the load sensing valve.

 

OK - I know this horse was turned into glue long ago, but I just want to be sure:

 

On the MJ distribution block, the port you plug to eliminate the rear height sensing valve is the one on the nose of the block, right? and the one on the bottom front is to be plumbed directly into the lines from the rear axle?

 

So many threads have come and gone ion this, and many pictures have been removed. There is no 'clear' picture of the entire system (as did exist some time ago).

 

Thanks!

 

All of the charts and pictures I could find had bad links.  Two questions: If I'm keeping the MJ distribution block, is the above assumption correct? plug the nose and keep the bottom front.  And since the flex line isn't long enough (nor have the right connections), can I just run a hard line extension from the one line that I'm keeping to the flex line?

 

If that's the case, then it sounds pretty easy (and cheap) and worth trying.

[Eagle]

All of the charts and pictures I could find had bad links.  Two questions: If I'm keeping the MJ distribution block, is the above assumption correct? plug the nose and keep the bottom front.  And since the flex line isn't long enough (nor have the right connections), can I just run a hard line extension from the one line that I'm keeping to the flex line?

 

If that's the case, then it sounds pretty easy (and cheap) and worth trying.

 

Nope. Just the opposite. Plug the bottom front, use the "nose."

[carnuck]

Or loosen the bottom front while bleeding. It's the return line and air gets trapped in it that can't get out any other way.

[Akula69]

Wow - my old brake problem has bit another MJ in the , ah - pedal!

 

I went through 4 quarts of brake fluid bleeding the brakes with a power bleeder over and over again when I finally posted that thread. We finally solved the problem by manually loosening the lines at the "T" under the bed and bleeding them. I kept the proportioning valve and the height-sensing valve from the MJ in the system without modification; solving the problem involved a second person disconnecting the lines at the "T" one by one and having the person in the cab push fluid out of them until they observed a solid stream (Obviously, with the line undone I would press the pedal down and hold it until he plugged opening with his finger, then release the pedal).

 

After that process was done on all three lines I have brakes like never before....

[Rymanrph]

I don't have any help bleeding, so I have to do it myself.  I think I'm just going to remove the valve and try that.  I can always put it back on later.

[AMCJeepMJ]

Thread bookmarked for when it comes time to bleed my brakes next. My proportioning valve is still intact and seems to still function.

[Rymanrph]

I removed the height load valve (whatever its called) and my pedal is a little better, but not great.  I think what's failing me now is my bleeding method; I'm doing it by myself with a tube down in a 2 liter bottle of brake fluid.  I don't think the tube is making as good of a seal on the bleeder as I'd want, so I think it may be pulling a little air in when I lift the brake pedal.  

 

I did find a loose hard line on the rear axle, so I know that probably helped.  I'm going to take the truck to be inspected next week and I'm going to have the shop take a look at the brakes while its in there.

[Jeep Driver]

Cut  you a 2x2 the length of the distance between the seat and the brake pedal when it is to the floor, you can use something between the seat and the 2x2 to protect the seat.....towel, old license plate.....whatever.

 

 

Open bleeder, depress brake with 2x2, use seat to keep the 2x2 pressing the pedal to the floor, then close bleeder.......repeat.

 

 

My wife usually helps with bleeding but I always find ways to do things on my own.

[HOrnbrod]

I never could bleed the brakes worth a damn by myself. Borrowed and tried pressure bleeders, vacuum bleeders, etc. The old PUMP-HOLD-CRACK BLEEDER-CLOSE BLEEDER-RELEASE PEDAL and start over again and again using the wife or whomever's available as the pumper and me as the bleeder works the best.

 

I've tried to use Gus the Cocker Spaniel, but his legs are way too short. Occasionally he manages to hold the pedal down, but when he releases it hits him in the mouth and he attacks the pedal.  :shake: 

[carnuck]

Cut  you a 2x2 the length of the distance between the seat and the brake pedal when it is to the floor, you can use something between the seat and the 2x2 to protect the seat.....towel, old license plate.....whatever.

 

 

Open bleeder, depress brake with 2x2, use seat to keep the 2x2 pressing the pedal to the floor, then close bleeder.......repeat.

 

 

My wife usually helps with bleeding but I always find ways to do things on my own.

 

 

I use a modified steering wheel lock. 

[Rymanrph]

The fronts aren't too bad since I have speed bleeders.  I couldn't find any small enough locally for the rear cylinders.  At this point, I just feel like I'm wasting brake fluid.  I haven't driven the thing in so long that every time I crank it to see if its working, I have to put a jumper on it.