Howdy, guys,
Been a year or so since I repainted and fixed up my truck for my wedding. Hope you're all well.

The truck has been running well and I even took it on a road trip again from WA to MN in January.

While on the trip, however, I noticed the brake pedal getting soft and going further to the floor, but not all the way. It only happens in town at slow speeds or pulling into a parking spot.

After some time attempting to diagnose it, here's what I know:
- no visible leaks
- only happens at slow speed and low idle but reving engine brings back pedal feel (vacuum issue?)
- fluid is good (couple years old and full)
- adjusted rear brakes which helped but it still does it (right rear wheel needed several turns)

I replaced all the brakes a couple years ago and installed new LD calipers, cylinders, shoe kit, etc.

The master cylinder and brake booster are ORIGINAL to the truck from 1999. Is it time? How do I diagnose the booster and / or cylinder? Why does it only happen at idle / low vacuum?

Everything I know or have read indicates at this point to take it to a shop. I don't want to. Thanks for your advice and hope everyone's doing well.
-Jeremy

 

Actually, at idle, you have lots of vacuum.

So, start the engine, let it idle, then disconnect the vacuum line going to the booster. (it should have a check valve on the booster, so, disconnect the other end.) Step on the brakes. You should be able to get two or three 'assisted' pumps of the pedal before it turns into a rock.

Have you replaced the line going to the rear? Mine popped, but, it took a while before fluid actually reached the ground.

 

Actually, at idle, you have lots of vacuum.

So, start the engine, let it idle, then disconnect the vacuum line going to the booster. (it should have a check valve on the booster, so, disconnect the other end.) Step on the brakes. You should be able to get two or three 'assisted' pumps of the pedal before it turns into a rock.
So, does this test the booster? Or what is the point of this?

Have you replaced the line going to the rear? Mine popped, but, it took a while before fluid actually reached the ground.
Nope, all stock lines.

 

The rear brake line that runs along the frame beside the fuel tank is famous for rusting out and leaking. Take a flashlight and check in that area to see if you have a pinhole leak starting in that area. That truck you have is 24 years old and those metal brake lines get fragile after all of those years. I would strongly suggest changing them out if you plan to keep the truck for any length of time.

 

Yeah, that's a booster test.

I would be having a real hard look at the line to the rear then.

 

So let me understand your test: if I disconnect the booster hose and pump the brakes, the pedal should get hard in 2 or 3 pumps if the booster is good?

What does it do if it's bad? What am I looking for here?

I know the lines are old but my truck has never lived in the rust belt. When it starts warming/drying up I'll look into new lines. Still trying to find the exact cause of this pedal issue. I don't use parts cannons anymore - only replace what's actually needed.

Thanks, guys. Good to see you're both still here.

 

My truck isn't a rust belt truck either, but that didn't stop a rear line from bursting on my '82 Dodge Ram and my '96 Dodge Ram. The way that rear brake line runs along the frame and beside the fuel tank near the rear wheel seems to have an effect on that line in particular over several years..

 

Yep, if it works as described, it's likely good. If you DON'T get power assist for a couple pumps, then either the check valve is bad, (assuming it works with engine running) or the bladder is blown, in which case, you wouldn't get power assist regardless. Boosters don't go bad often, and its usually the result of a failed master cylinder letter it suck brake fluid in.... it does a real number on the rubber bladder.

 

Hmm, okay, thanks guys. I'll do some more checking. Having actual winter weather out here this week so once it dries up a bit, I'll take a better look.

When I figure it out, I'll be sure to finish the thread.

 

Booster test - got stiff in two pumps

Again, the pedal will soften and sink at regular idle but when I raise the idle just a little, it gets normal strong. Why does this happen?

Also inspected the rear brake line. Looks crusty but no visible leaks. If there's even a pin hole, there will be fluid leaking. It's impossible to check it properly unless I drop the tank anyway.

Who knows.