Steve@Advance

Steve I have a 72 GMC 5500 series truck that I needed to replace the Vacuum booster on. How do I figure out what unit to order? I don't want to remove until I have new parts in hand.
Thanks Bill in Colo

Need to gather all the info first.

Is it a firewall mounted vacuum unit, or a remote Hydro boost? Drum brakes or disc? Brand of brakes, Bendix, Delco, etc.

You can look online, compare prices, shipping charges, sales tax. Then give them a call, see if they need any other info, like brake size, axle capacity, etc.

Or, if there is a truck friendly auto supply near you, have them look it up, or take it by, let them look at it.

Typically, if it's a firewall mount, you will get a rebuilt booster/master cyl combo. That way everything works together. It may not look exactly like your old one, but if it will connect up it should work.

Just to be sure, have you checked the vacuum to the unit? The old hoses were prone to collapse inside.

If I may butt in here, if it is a remote (frame-mounted) booster it will have an ID tag attached to it with a # that a supplier will be able to cross to the brand they sell, typically hangs from the bolt that cinches up the band around the diaphragm chamber.

(Unless some "helpful person has removed it!)

If it a "medium duty" truck, it's gonna be awful tough to look a replacement booster up by application 'caus there are so many variations depending upon how the truck was originally spec'd out and built frame, spring, axle and brake-wise.

So, I hope you get lucky and find a tag!

Steve the Unit is remote frame mounted. We don't have anyone around anymore who knows about older trucks.

Well you may just have to remove it and send it off to get rebuilt ?
Hemmings motor news may just list such a place to do the work.

That is called a "Hydrovac" unit. Yes, they are dinosaurs! Odds of finding one rebuilt and ready to go are slim. Like Bob said, look for a tag with a number. You'll need to positively identify it to get the right one. Some look the same, but the insides need to be right in order to work.

There are listings on Ebay for rebuilt ones. They may be able to help, but best to have some numbers or brand name available. If you don't want to deal with Ebay, start expanding your search to the larger cities. There should be a truck brake specialty shop that can help with getting a rebuilt one or rebuilding yours.

How is the rest of he brake system? These boosters are not pleasant to deal with, difficult to diagnose! Be sure everything else is up to par before installing the booster. If you haven't already gone through the rest of the brakes, I would pull all the drums, replace or rebuild all the wheel cylinders and the master cylinder, check the lines for rust and rot, flush everything, reassemble and adjust so you know it's all good. Sure don't want to contaminate the rebuilt booster with dirty fluid or be chasing a problem elsewhere in the system!

what makes you think the booster is Bad, their are so many variables that would make you think its the booster, I have worked on many, brakes out of adjustment,, collapsed vacuum hoses, might want to chk that first