Brake troubleshooting

[trout14]

First time posting on here. Recently picked up my first MJ and have been doing repairs to get it road ready.

 

Spent the day trying to get the brakes working better and am wondering if I'm on the right track. When the engine is off pedal is solid but once started/driving it losses all pressure and needs to be pressed right to the floor for it to come to a full stop.. thinking bad booster? there's vacuum right up to the housing.

 

 

[ghetdjc320]

The booster is just a pedal assist so it sounds like that is working great as it only worked when engine vacuum is present. I’d troubleshoot elsewhere in the hydraulic system. Start by examining carefully for any brake fluid leaks. How is the fluid level in the master? Did it ever run low or even dry? 

[trout14]

36 minutes ago, ghetdjc320 said:

The booster is just a pedal assist so it sounds like that is working great as it only worked when engine vacuum is present. I’d troubleshoot elsewhere in the hydraulic system. Start by examining carefully for any brake fluid leaks. How is the fluid level in the master? Did it ever run low or even dry? 

Fluid has been consistent. Had it up on a 4 post yesterday and check every inch of that linbe. No leaks but it is rusty. 

[cruiser54]

Make sure the rear shoes are adjusted properly after inspecting them. 

[Pete M]

exact symptoms of my 88.  replacing the booster fixed it.   

[ghetdjc320]

There is a check valve in the booster that should hold at least one full pedal assist pump after the engine is off. Try starting the engine, let it run at idle for

just a bit then shut it off (don’t touch the brake pedal during this time). Give it about a minute then try pushing the brake pedal. This will tell you if the booster is still holding vacuum as intended. 

[AZJeff]

The standard test for a vacuum operated brake booster to verify functionality is this:

 

1.   with engine OFF, pump brake pedal repeatedly until the pedal feels solid

2.  while HOLDING pedal down, start the engine.

3.  as the engine begins to run and generate vacuum, the brake pedal should "sink" further with existing foot pressure

 

If it does NOT do this, the booster is defective (assuming the hose from the booster to the manifold is connected and not cracked).

[trout14]

4 hours ago, ghetdjc320 said:

There is a check valve in the booster that should hold at least one full pedal assist pump after the engine is off. Try starting the engine, let it run at idle for

just a bit then shut it off (don’t touch the brake pedal during this time). Give it about a minute then try pushing the brake pedal. This will tell you if the booster is still holding vacuum as intended. 

Tried that and it took a couple pumps to get the pressure back up after stopping

[ghetdjc320]

Then your booster should be working properly. When you feel the “pressure come back up” that’s because the booster is no longer assisting the pedal effort and you’re back to manual brakes. Pete said the booster fixed his issue . I could see you needing to replace the master cylinder though. 

[ratrapp]

my rear prop valve was bad on mine.i never could get a good pedal until i bypassed the valve.i replaced wheel cylinders,master cylinder,hoses and lines until i finally bypassed the valve.

[gogmorgo]

X2 on what Cruiser said. Make sure your brakes are working properly at the wheel end before getting fancy. It also sounds to me like there’s nothing urgently wrong with the booster. 
I try to change all the pads and rotors (or shoes and drums) soon after a new purchase, even if they seem alright. Mostly this is so I know I have reasonable quality parts on the thing but also because it gives me the opportunity to make sure everything’s working. 
But even if everything is working, drum brakes are never adjusted properly. You have to stop in reverse to work the adjusters, but no one ever drives fast enough or stops hard enough in reverse for them to adjust. Easiest way to adjust them if you’ve got an empty parking lot or somewhere to do it safely is to get up to 15mph or so in reverse and stop firmly. Repeat a few times. If nothing changes you’re going to need to either crawl under and adjust them manually or else pop the drums off to make sure everything is kosher.

 

Something that isn’t specifically unique to the MJ but isn’t super common in the automotive world and does cause confusion is the rear load sensing proportioning valve. From the factory you’ve got a valve in the brake system that increases rear braking as the rear suspension squats, basically as you add weight. It sits on the driver’s side of the rear diff, there’s a flat bar on the valve with a rod that extends down to the diff, ball and socket at each end. The bar should sit just above horizontal, and the rod should be vertical. It’s common for the rod to go missing or for the two to get flipped so the bar is pointing down and the rod is at an angle. You should be able to pop the rod off one of the sockets to put them back, or tie the rod up in roughly the 2-o’clock position. If the truck is lifted, the rod should be extended accordingly. At any rate, if the bar is sitting anywhere below horizontal your brakes will not feel adequate. 

Something else kinda unique is there’s a bypass line for this valve, that’s supposed to send full brake pressure to the rear brakes if there’s a failure in the front brake circuit. It dictates an extra step in the brake bleeding procedure. You open the bleed screw on one of the front calipers to simulate a front brake failure, and leave it open while you bleed the rear brakes. Then you close the front bleeder and bleed the brakes as you normally would, by bleeding the rear brakes a second time, and then bleeding the fronts. 
 

One you’ve got the basics worked out, then we can move on to things like master cylinder and booster. 

 

[trout14]

Thanks for you time on the reply, I’ll be sure to take a look at all of that before going for the booster