1985 6.9 IDI, E-350, Dually, 14 ft Box, auto. 150,000 miles.

I am having issues with the brakes. After getting my head under the hood, and remembering that I am on a steep diesel learning curve, I realized that the booster is powered by a belt driven vacuum pump (DUH, no vacuum on diesels).

Transmission shifts fine, really smooth actually. No clunking in to gear, or through the gears.

So, I've got some squishy brakes. This I'm sure is air/master cylinder.

I also, intermittently, get heavy heavy brakes. They work, but the booster is clearly malfunctioning.

How do I check to see if this is vacuum pump, or the booster itself.

I am nervous to mess with the vacuum pump, because I don't want to mess up the transmission. I believe this is a vacuum driven auto trans.

Tips?

Thanks folks. I should mail all of you a six pack when I get the first month maintenance free on this beast.

-Hugh Jones

 

I plan to start by swapping out the belts. I figured it might be as simple as a wobbly V-belt not driving the vacuum pump, I doubt it, but it needs to be done any way.

 

In my experience, when a brake booster starts to go bad, it will making a hissing noise when applying the brakes long before it actually stops working completely.

I would start with the vacuum pump. Remove the vacuum line from the brake booster and connect it to a vacuum gauge. HF sells these gauges if you don't have one. The vacuum pump is supposed to maintain something around 21 in Hg, so if it produces anything less than that, it's suspect.

 

Just curious, what is the probability that I screw up the transmission when I do this?

 

Also, is this at idle? Does it matter what RPM the engine is at to get a good read on vacuum?

 

I don't think you have anything to worry about as far as the transmission is concerned.

It should produce 21 Hg vacuum at idle. In my factory service manual, there is a chart that says exactly what amount of vacuum you should see at what altitude. It also has the diagnosis procedure for the vacuum pump and booster. I can scan that later and post it.

 

vacuum will be the same at idle or at full throttle. anything less that 19 inches and the pump is bad.
if your trans is shifting perfect, i would say the vacuum pump is good and the booster is leaking internally.

 

Alrighty. I hooked up the vacuum tester (A bosch model from O'reilleys $25). I am reading 24 in Hg from the main vacuum line. I am at sea-level (Olympic Peninsula), as it seems altitude matters.

Does this hissing happen inside the cab? I just realized that the brakes do this every time I apply them. Seemed normal, so I overlooked it. It is becoming clear to me that this is not the case. (Excuse me for being an idiot, I am learning here.)

 

It seems that you are definitely correct here. I went ahead and bought the tester, and tested the main line. I am getting 24 inHg. Seems like the vacuum pump is in good shape, eh?

Guess its time to buy a booster and master cylinder.

 

Also, thank you all.

Really, I appreciate the help. You guys are helping me get my business running (literally), and I couldn't do it without you. I am trying to start this business without going into debt, and that requires developing my mechanic skills.

Thank you.

 

Yeah, sounds like it's the booster then. I think the hissing would likely be more noticeable in the cab. Also, since you said you had squishy brakes too, it's possible that the master cyl was already leaking and got fluid into the booster and made it go bad.

 

I believe the fitting on the vacuum boost is also a check valve. Or, the rubber fitting could be leaking, or it could be leaking at the MS connection or the pedal connection. Sometimes you will hear a hiss in the engine or cabin. You might have to turn the engine off to hear it in the engine compartment. Typically, if a MS/Vac boost is working correctly, you can pump the brakes with the engine off to deplete the vac, hold the pedal while starting, and the pedal should drop and stop without the light coming on.