OK. I thought I knew how to do this stuff, but apparently I'm missing something.

1985 F250, with 6.9 diesel.

A while back the master cylinder went out on the old truck. I figured this was no big deal, bought a new one from Oreilly's, and installed. I thought I followed the directions for bleeding the master cylender while installed. I pumped the brake a lot, only moving it in very short strokes of the foot pedal. Also, before this, I was leaking fluid out of the rear brakes, so I rebuilt both rear wheel brake assemblies using rebuild kits, bought new (crap ... can't think of the name) piston thingies for them, and adjusted properly.

After that, it just didn't seem like I'm breaking good. I bought a mighty vac, and bled all brake lines using it, but I still brake the same way.

When I press the pedal, I get almost all the way to the floor before you feel it start to break. If you continue, and press as hard as you can with you foot, you can get a pretty good brake, but the pedal is almost on the floor and you have to press as hard as you can to really feel like the brakes are going to stop you.

Also, occasionally, while driving down the freeway and not breaking at all, the brake light on the dash will illuminate. As soon as you get off the freeway and press on the pedal, it usually goes away.

What did I miss? What am I doing wrong? I thought I knew this stuff pretty well, but am having trouble diagnosing this one.

 

Sounds like either air is still trapped in the system, or the replacement master cyl just isn't good. New or reman does not always mean good, I've found that out the hard way, many times over.

Did you "bench bleed" the master before installing it? Skipping this step can lead to having air trapped in the system that is *nearly* impossible to get out.

 

I didn't bench bleed it. At the time, I didn't have the mighty vac. The instructions told how to bleed it in the system instead of on the bench, and I followed those instructions pretty good. Did I miss that step?

 

Did it come with the plastic fittings and hoses to bleed it before attaching the lines? If so, and you did that, it should have been fine. I do that method myself.

Also, are the rear shoes adjusted properly? If they are set way too loose, it would cause the pedal to be rather low before any real braking action started to occur.

 

The easiest way to bleed out that system is this, Bleed the master on the truck... get a pair of Metal lines that go to the master, make them or getting them from a parts truck somewhere is easiest, bolt them into the master as you normally would and bend them up and into the master deep into the fluid level and pump your heart out until all the air is gone... simply recconect the orig lines from the truck and begin normal bleeding procedure starting with the right rear wheel .. I find this as the easiest method and havent actually bench bleed a master in years...

 

The plastic fittings and rubber hoses that are *normally* included with a replacement master cyl will do the same thing described above, but simpler to use.

 

Hmmmm ... it came with the yellow fittings on it, but no hose. It had a book thing with it that said the two ways to bleed it. One was bench bleeding, and the other was to install it, and press the pedal 40-60 times at less than 20 degrees. Then another bunch at 40 degrees, and on and on till you were pushing the pedal all the way down.

I've never done a bench bleed. How is that done?

 

Yea but those stupid rubber hoses always pop out and rarely are the damn things actually long enough... LOL

 

Bech bleeding is exactly the same procedure its just much more of a pain in the @ss as you are securing the master in a vice and pushing the plunger manually with a screwdriver or socket extension....

 

Okay ... I think I got it. Sounds like I'll try that and see if I get some improvement.

 

One more question. Can I just use the mighty vac to suck the air out? I'm assuming the answer is no, since I used it to bleed all my brake lines and it still acts this way.

 

I have never had much luck with those vaccume bleeders... I have had lots of luck with pressure bleeders which attach to the top of the master and push fluid through the system, just make sure you keep the master full as you will chase your tail forever...

 

Thanks thndrroad and Rogue! I'll try bleeding that and see if I get improvement.