I replaced the master cylinder this morning, (it felt like it was bypassing) bench bled it before putting it on the truck. I also bled it on the truck. I've bled all the brakes also, pedal is still going all the way to the floor. If you pump it up, it holds pressure until you either release the brakes or start the truck. Could this be the booster?

 

Yes, it probably is the booster or you have a leak in the break lines somewhere.

 

Checked all the lines no leaks, and the wheel cylinders are clean. What about the vacuum pump? I'd rather replace it than the booster.

 

Of course, you know that the 1st order of business is to be SURE that the rear brakes are 100% adjusted until thay have a light, but noticeable drag.

If the pedal is hard, then sinks to the floor when you start the truck, it's likely not a vacuum problem.
There's a hydro leak somewhere, most likely internal.

Beware, some of the reman master cylinders are junk.
Several folks have had issues with the reman masters.
One fellow even went as far as to take his reman master cylinder apart, just to find that the bore hadn't even been honed as part of the reconditioning!

Finally, procure a vacuum gauge and check that you have sufficient vacuum, at least 15", to properly run the booster.

My number one thought though, is adjustment and/or hydraulic leakage with air in the lines being a possibility.

Mark.

 

If the booster is bad, then the pedal is "ROCK HARD". Don't spend money where you don't need to. These brake systems are temperamental on how they get bled. I've done what you're trying to do several times, and I wasted a lot of money buying parts I didn't need.

Master cylinders should be purchased as "NEW" and not reman. I went through remans, and new Napa units, and all of them either leaked internally or poured fluid out the cap. It wasn't until I bought a brand NEW bendix unit, that I finally had a normal, functional braking system..... after bleeding it properly, that is.

http://www.amazon.com/Bendix-11775-M...1&s=automotive

I will never buy another Napa master cylinder after going through what I went through with their junk.

You need to fabricate the bleed recover container found in numerous threads before this. It's the only real way to make these systems fluid tight and air free...

 

Trust me, from my own prior bad experience, trying to bleed the brakes with the rear brakes improperly adjusted is an excercise in futility.
They MUST be hitting the drum solid before they will bleed fully.
Best way to bleed brakes by yourself is a piece of vacuum line into a partial bottle of brake fluid.

Mark.

 

Precisely what I'm talking about.... You need pressure of the shoes hitting the drums during bleeding.... and this too.

https://www.dieseltruckresource.com/...6&d=1320054923

 

as stated, a vacuum loss would make your pedal HARD, remember, the vacuum assists in braking, without it, your on manual brakes....

Did you bleed the RWAL Valve located on the frame rail, drivers side, above the rear axle? If not, you wont get the air out of the system. You'll have to bleed them all again, starting with the RWAL Valve

I replaced my MC two days ago and found this TSB on brake bleeding. Ignore the parts about the front anti-lock valve and combo valve, but the rest is correct for our trucks.

https://www.dieseltruckresource.com/...n-t310768.html

 

This statement tells me the pedal is going straight to the floor, WHEN HE STARTS THE TRUCK or RELEASES THE BRAKE.
I'm gathering from this that the brakes lose pressure once released, which is usually air in the system.
It's likely NOT the vacuum booster, but rather the adjustment of the brakes, or the fact the air is not bled out properly.

The other possibility is that the master cylinder is leaking internally.

Mark.

 

I bought a brand new master cylinder from napa his morning, and already ordered a booster also. Oh well. The factory ones were wore out and the old master cylinder leaked anyways. As for the rwal valve I wasn't aware of it, I'll bleed it tomorrow. I did bleed the proportioning(?) valve and the other little valve thats on top of the drivers side frame. The rear brakes are adjusted tight, but I will check them again. Thank you guys for your help! Ill keep you updated.

 

I helped a buddy on a 93. Hours of bleeding with no luck. By passed the rwal and it helped, but not good enough. He threw up his hands and bought napa new front rotors, calipers, pads, and brake lines, new rear drums, shoes, brake cyl.. I went back and helped adj everything but it was only slightly better. So he bought napa new booster/mc matched set. After a 1 1/2 gallons of fluid he had decent brakes. Find a hydrovac off something and install because IMHO you will save headaches and $$$$. My understanding these ole trucks had only fair brakes at best new.

 

I disagree, after replacing a leaky MC, and after properly bleeding the system, it'll put ya up on the dash if you stomp'em..and I know I need to replace teh pads and shoes yet..
The brakes on these trucks are good, IF they are maintained and adjusted properly. the only real weak-link is the auto-adjusters, which dont work as they should once they get a touch worn down.. but manually adjusting the rears every oil-change or so is easy.
The design of this system makes it hard to bleed properly for some reason tho, and if you go back through the posts in this forum for the last few years, that is the biggest issue on them. Most folks forget or dont know about that pesky RAWL valve and bleeding it before the tires, or how much fluid it takes to run through all the blasted brake-lines on one of these trucks and so dont do a thorough job.


just my 2cents worth

 

I see your 2 cents worth, agree with the call, and raise you another 2 cents.

My truck stops great, and I just ordered all new rear braking parts (shoes, drums, springs, RR brake cable, seals, etc). I upgraded to 12x3 shoes last year, but never touched the drums. THe drums were on their way to the scrap yard, and I just did the hydraulics (new 1 1/8" wcly's) and shoes, and threw it back together. This time, I have all new parts, including a new RR E-brake cable that I replaced with a new one last year, but is malfunctioning china crapola unit. hopefully the new raybestos unit is better.

 

THe new (reman booster) from napa will be of no harm to you, and at 20 years old, it won't be a bad idea to have replaced.... as it just leaves one more thing that won't have to be looked at for a while.

The new Master Cylinder from Napa leaked continuously from day one on my truck. The rubber seal under the cap is made of a very hard rubber that won't seal against the machined surface of the reservoir. They gave me a new seal, then a new (new) master, and nothing worked. Eventually, I gave up and bought the Bendix unit from amazon. Never leaked at all. The seal is more supple / soft, and allows complete sealing of the chamber. When both seals were held next to one another, there was a remarkable difference. Again, it's not leaked at all since installation.

I never touched that proportion valve under the hood, as the FSM didn't specify to do so. If the valve is in the right position (under the little rubber cap), then there's nothing to do but bleed the system as required. I cannot tell you how many hours I spent ...... but there are a few threads previously that explained what is necessary to get it right.

 

I've been checking out all the threads today, Im gonna try bleeding it starting with RWAL first, then work my way to the tires after adjusting the rears. Sidenote: When I pulled the Booster of off my truck, it had brake fluid inside it from the old MC leaking out of the back, its probably a good thing I'm replacing it. Im gonna install it tomorrow!