Time for some power brakes! In this sixth installment of the Loadstar Project I’ll be addressing the power brake booster, sometimes referred to as a “hydrovac”. Into the shop it goes…


Now this has been one of the more “pain in the neck” repairs I’ve done to this truck. First off, I knew the main diaphragm in the brake booster was bad. I had a similar booster from my F-700 with a stuck piston but a good diaphragm. I was desperately hoping all I needed for mine was the diaphragm because parts for these, specifically an overhaul kit, are expensive and hard to find. If I could switch out the diaphragm with the F-700 unit, it would be an easy fix. These boosters are not even made anymore, and although a remanufactured booster can be had for around $250.00, I really hadn’t budgeted that in to this project.
So once I had it in the shop I rolled under the truck to begin. The lines and fittings looked rusty and hard to remove, so at first I thought I would try to repair the booster with it mounted on the truck. I removed the main diaphragm cover to reveal, as anticipated, a broken diaphragm. What I did not anticipate was that half of the chamber would be full of old brake fluid which splashed everywhere and was the actual cause of the deteriorated diaphragm. Now, bare in mind this is supposed to be a dry chamber, so the fact that it is full of brake fluid reveals a more sinister problem than a broken diaphragm. It means the piston rod seal is leaking fluid from the cylinder into the diaphragm chamber. This explains why the previous owners told me they were having to add fluid, thinking it was a leaky wheel cylinder.
Time to find an overhaul kit. I searched high and low and found nothing. Finally, I happened across a brake parts remanufacturer’s website called remaninc.com. They had a series of kits in their online catalog, so I just needed to figure out which type of booster I had and locate the correct part number. Like all boosters are supposed to have, there was an aluminum tag on the unit with a series of numbers. These numbers identify the brand and model of booster. I made note of my numbers and searched the website for the rebuild kit for my unit. Unfortunately, my number didn’t pull up anything. However, as a last resort, I called the phone number on the website and they actually have a parts counter for customer support. Surprisingly, the guy I spoke with knew exactly what I had and exactly the correct kit I needed, which they had in stock by the way, and he shipped it out that same day. If I remember correctly, I think the cost was about $85.00. I couldn’t be happier.
So trying to do this with the unit still on the truck, I now have to remove the steel brake line and the end of the cylinder which is a large nut as seen below, so I can slide the piston and rod out in order to replace the leaking seal:



I went ahead and removed the smaller diaphragm on the bottom along with the springs and other components as everything would have to be cleaned thoroughly and seals replaced.

Now I have to remove the main shaft seal and “retainer” for lack of a better word. This is where the problems begin. The seal and retainer are pressed in. Once the piston was out, I removed the appropriate snap rings and tried to use a punch to drive out the retainer. Fail. I hit it with everything I had and it didn’t budge a bit. It was at this time that I determined I would have to remove the entire unit from the truck so I could put it in my hydraulic press. Below you’ll see a photo of the chamber side. In the very center is the part that I have to press out from behind once the snap ring is removed.

To fit the part in my press, I remove the chamber housing and put the cylinder in the press as shown below:

I didn’t have much problems pressing out the retainer and spacer, but the parts being plastic were slightly damaged. Unfortunately, the kit did not come with replacements so I’d have to repair these and reuse them.

So now that I had the entire unit disassembled, it was time to clean. No easy task. As seen below, the parts were heavily rusted. I could only hope the cylinder would take a rebuild, otherwise I just wasted $85 on the kit.

I washed everything in my parts washer and air dried it before bead blasting in my blasting cabinet. I laid out all the parts and the rebuild kit along with the instructions.


I polished the piston and rod with a wire wheel so the seals would travel smoothly. Then I installed the piston cup seal. I also honed out the inside of the cylinder with a brake cylinder hone.

I pressed the seal retainer with the new rod seal and o-ring into the cylinder. Then the entire unit is reassembled the same way it came apart. Now it’s ready to go back on the truck.

Once back on the truck, I manually bleed the system. When adequately bled, the moment of truth arrives. It’s easy to tell if the diaphragm portion works. I start the truck and if the engine dies when I push the brake pedal, the diaphragm is bad. If it stays running and I hear a vacuum sound, it’s good. As for the cylinder portion, I would have to drive the truck to test it.
The verdict: I start the truck and push the pedal. The engine stays running and I hear the booster working. So far so good. I drive the truck out of the shop and hit the brakes hard. The wheels easily lock up. Success! I drive it around the yard and test it thoroughly. I then re-check my brake fluid level. Looks good. The only unusual thing about it is that the brakes are a little slow to release. Other than that, everything works. I now have power brakes on my Loadstar!