Booster is probably ok then. Done any brake work to the truck lately? Might be tempted to pull the rear drums, and see what the wheel cylinders look like.

 

Like I mentioned, I replaced everything a couple years ago. Looks good but just used regular parts store stuff so could be bad I guess. Nothing is leaking though.

Already took drums off, cleaned, inspected, and adjusted everything. The pedal is weirdly inconsistently acting up. Sometimes great, once in a while it sinks and I double pump it. Especially in parking lots.

 

Another way you can test is run the truck for a minuet and shut it off, Then press and hold the pedal down as if you would coming to a stop. If it sinks the master cylinder is bad. If it's good it will hold pressure until the vac runs out. Even without vac assist it should hold the pressure and not sink. Master cylinder is what it sounds like IF you don't have leaks anywhere.

 

Thanks, Moparite, I'll do that test the next time I run it. I'm wondering too about the Master. The only components that are original (besides lines) are the MC and booster.

 

Did your test, Moparite, and it held just fine.

 

Checked the following as well:
- engine vacuum steady at 17-18 inches
- no visible leaks on booster, hose, or check valve
- tested check valve at booster = good

I'm thinking it may be at the calipers or cylinders but why would raising the idle fix the soft pedal? It must be an issue at the booster, right? More throttle equals less vacuum, right?

 

Disconnect the master cylinder from the booster, and separate 'em a bit. No need to disconnect any lines. See if you are getting fluid out the back of the master.

 

Pulled the MC and fluid ran out and down the booster. I suspect it's been leaking for years since the booster was rusting from the bottom but I figured age or salt or whatever. .

Wonder if it started rotting out the booster diaphram or merely finally got bad enough to notice. Considering the RPM changes the pedal feel, it must be in the booster.

So I'm shopping rockauto and wondering if there's anything I should know. Looks like SKP is the only non-reman'd (new) booster available. Any recommended MC brands or something to avoid?

I'll put off the lines until spring when I can spend time on the whole truck and can use the bikes for transport.

Thanks for the advice and let's hope this fixes it. Considering the condition, even if the problem persists, these parts needed to be replaced anyway.

 

Go with one of the "major" name parts. Skip the cheapo rebuilds... Dorman, Cardone, etc. If you can get a new one, as opposed to a reman, go that route.

 

What's the difference between 1999 and 2000 brakes boosters? There's different parts listed. I like to check other years in case something changed or was upgraded.

I know the calipers changed in 2000 but the MC parts show the same. The boosters say "for LT265 or LT 275 tires" in 2000 and have completely different part numbers. No such description in 1999.

Did they up the power? Different size? I've already upgraded to the LD large piston calipers and if I could swap a more powerful booster I'll do it. Plus it's cheaper than my 1999 part.

Any ideas? Again, same master cylinder is listed. Can't imagine they wouldn't interchange. If I can upgrade something, especially an easy one, I'll do it.