I am currently trouble shooting my brake system. The symptom is that the pedal travels just about to the floor when the brake is applied. I have a full reservior, system has been bled, and the brakes are adjusted. I recently consulted a professional diesel technician and asked him what I could do to troubleshoot the system. He said to get some pipe-plugs and plug off the outlet ports of the master cylinder. Doing this would isolate the master cylinder from the rest of the system. This test would also determine if the master cylinder is bad or not. I have read way too many stories on DTR that involve folks buying a new/reman MS and not fixing anything.........I don't want to be a person to just buy and throw parts at it until it's fixed. Today I installed the pipe-plugs on the master cylinder, started the pickup and tried the brakes. The pedal went down freely halfway and stopped abruptly. I kept pressure on the pedal and it did not go down anywhere past halfway (solid as a rock). The thing that really surprised me was the "free-play" on the pedal. Upon applying the brakes, the pedal very easily goes down halfway and then hits solid pressure (no bleed-down). I'm convinced by this test that my master cylinder is in good shape. If it were bad, the pedal would slowly creep to the floor. The thing that worries me is the pedal "free-play" until it acually starts applying the brakes. How far is the pedal supposed to "free-play" until it starts to apply the brakes? I would appreciate any insight on this subject.

 

It sounds like the rear piston in the master isn't doing anything - or the pushrod in the booster is way too short.

 

Had same issue with my 89,
Bleed those rear brakes the old fashioned way,
One working the brake pedal, the other on the wheel cylinder plug,
When you think youre done, do it more,
It took a bit if time for mine to come up good, After I was done and brakes felt really good I was driving on the highway and had a small panic,, Hit the brakes like I used to, and locked all wheels @ 60 mph, Then I knew all the air was out.
Peter

 

Could be air in the master but I would also double ck. the booster pushrod to master cyl clearance. You want ABOUT .010-.015 or so. Dont get it too tight tho or your brakes will drag REAL bad.