Vac Pump or Brake Booster?

[big van]

My wifes van, 88 E-350 7.3 NA, has an occasional hard brake pedal.

The cruise seems to work ok but sometimes when you push the brake pedal it down shifts to 2nd and the pedal gets hard. Other times the pedal is hard with out it downshifting. I haven't pulled the vac line off the booster to see if it makes a difference or not yet. The pedal is only hard sometimes. Although more often tonight then before.

On the other hand when you push the brake pedal you hear a whooshing air noise. I've heard if the bladder in your booster is bad it will sound like that.

Anyone have an idea? I'd like to have a direction to look before I just start replacing parts.

Thanks, Matt

 

[gatorman21218]

best way to tell is to stick a vacuum gauge at the booster then at the pump itself to rule out any leaks. the gauge should read minimum of 21 in/hg at idle. your cruise and transmission both run off the pump so that makes the pump suspect

 

[Diesel JD]

Does anyone have a diagram of the entire vacuum hose routing for the IDI? I have had the same problem as the OP and it is scary for sure trying to stop that big a truck without any power assist. I eliminated the vrv and auto tranny so I think I may have a few leaks or open places in my system, also a thing to check on the 88 van. The vacuum pumps on these trucks don't seem to want to last. I've had my truck for 7 years and about 65,000 miles and have been through 2 or 3 already. Jim's right the vac. gauge tells the story but a leak or a disconnected hose could also be the culprit. Someone here has posted that most VP failures come from lack of lubrication internal to the pump and posted a way to fix it in most cases. If memory serves that was Damen aka Midnightrider. But to make a long story short your first step should be to check all vac hoses and get a gauge on the pump. With the auto you'll notice poor braking long before you'll notice shifting problems and low vac at the VRV so much below that 21 inches of vacuum is no bueno.

 

[LCAM-01XA]

JD, no diagram but I can tell you where the lines go:

1) 3/8" line fro the vac pump to a vac tree on the firewall
2) small plastic line from vac tree to inside the cab (if you have factory A/C)
3) 3/8" rubber line from vac tree to brake booster
4) 3/16" or 1/4" rubber line from vac tree to VRV
5) 3/16" or 1/4" rubber line from VRV to a steel vacuum line leading to the line pressure modulator on the rear passenger-side of the trans
6) short U-shaped piece of 3/16" or 1/4" rubber line at the very end of the steel line, connects the line to the pressure modulator
7) 1/4" or 5/16" rubber line from the vac tree to the vacuum canister on the driver-side fender (if you have cruise control)
8) 1/4" or 5/16" rubber line from vac canister to cruise servo
9) 1/4" or 5/16" rubber line from cruise servo through the firewall into the cab to the vacuum dump valve on the brake pedal bracket.
10) short U-shaped piece of 1/4" or 5/16" rubber line from one port of the cruise servo to another port on that same servo.

Think this about covers it... Since you converted to a bolt-action setup, you can safely eliminate #4, #5, and #6, and cap the port at the vac tree where #4 started from.

 

[MIDNIGHT RIDER]

My wifes van, 88 E-350 7.3 NA, has an occasional hard brake pedal.

The cruise seems to work ok but sometimes when you push the brake pedal it down shifts to 2nd and the pedal gets hard. Other times the pedal is hard with out it downshifting. I haven't pulled the vac line off the booster to see if it makes a difference or not yet. The pedal is only hard sometimes. Although more often tonight then before.

On the other hand when you push the brake pedal you hear a whooshing air noise. I've heard if the bladder in your booster is bad it will sound like that.

Anyone have an idea? I'd like to have a direction to look before I just start replacing parts.

Thanks, Matt

A dash-mounted vacuum gauge costs very little and is probably the simplest gauge to install.

Once you have the gauge, then most of the guess-work is done away with.



That being said, a bad booster will feel like it is pushing back at you when you try to apply the brakes.