Hydro-boost Brake Conversion on an F-series Pickup

By Full_Monte (Oilburners.net)

I’ve been unsatisfied with the standard vacuum-assist braking system on my ’85 F350 diesel for a long time. With a heavy camper load on the truck, I didn’t like the minimal power assist to the pedal. I’d read a couple of posts online by Towcat and finally saw his setup. With his advice, I bought a used hydro-boost out of an F-450 to make the conversion. Here is what I’ve learned about it, with some advice for those considering doing it themselves. I think this article would be applicable to most F-series trucks with the original vacuum-assist braking system.

The first thing is to understand what you are about to do. You will be removing the master cylinder and vacuum assist unit from the truck in one unit. To do this, you will need to disconnect the assist unit from the brake pedal inside the cab. Then you are going to replace it with a hydro-boost from a compatible-year truck and do some plumbing modifications. You will be running your hydro-boost assist system off the hydraulic pressure from the power steering pump. The way this works is pressure from the steering pump is applied to the master cylinder when the pedal is depressed. To get this pressure to the hydro-boost unit, you need the flexible hydro-boost hoses that came out of the F-450 truck. These parts cost me $250 from a junkyard specializing in larger trucks. I think you could do a lot better on the price than this by looking around.

Figure 1 Brake Pedal arm, pin, and actuator

Figure 2 Original vacuum assist & master cylinder

The pin in the brake pedal arm must allow the master cylinder shaft to rest horizontally when installed. If it is different, you may have to look for a different brake pedal with the pin hole in the right place or put a different hole in your brake pedal.

The steering pump originally had a pressure hose connected to the steering gearbox and a low pressure return line back to the steering pump reservoir (after going through a simple cooling loop line on the front of the cross-frame). The hydro-boost will need a pressure line and a return line to the steering pump as well. So, the basic idea is to hook the pressure side of the steering pump to the hydro-boost, connect the low-pressure return line back to the steering pump, and take pressure off the hydro-boost via another pressure hose to drive the power steering at the gearbox. (Note that you will need line wrenches to work on the line nuts. Regular open-ended wrenches will round off the nuts without removing them). The two pressure hoses going into the hydro-boost had me confused until I realized that it was basically a T-connection with the full pressure of the pump going through both hoses with some bleed-off for the power braking system. The only problem with all of this is that the steering pump on the truck has only one connection for a return line and you will have two return lines to hook into it. There are two ways to do this. One would be to tee both return lines together and then run one line back to the power steering pump reservoir. The other would be to buy the plastic reservoir for the F-450 pump for about $35 from the dealership and replace the part on your existing pump. That’s what I did.

In order to do the installation, you will need to remove the steering pump from the truck, install your new reservoir on the back of the pump, re-install it and hook up your new plumbing lines. There is an o-ring seal that will come with the new reservoir, which you will need to install carefully. I painted the new reservoir black, like the original.

Figure 3 Steering pump and new reservoir Figure 4 New reservoir part number

The plumbing lines on my truck caused me a few problems. I hooked them all up, not paying any attention to the ends of the lines. They seemed to fit. When I did the test drive, I left a trail of steering fluid all over the road. It turned out that the line coming from the hydro-boost to the power steering pump had a flare fitting, while the original line had an o-ring fitting. So, even though they used the same-sized attachment nut, it wouldn’t seal. The solution was to cut the end off the original hose, do the same to the new (longer) hose and connect the original end onto the new hose. A compression fitting handled this nicely after a couple of additional cuts on the metal lines to make it compatible for length. I could have bought a new hose, but $5 for a compression fitting was cheaper. I replaced the o-rings on both ends of the hose with #9 o-rings from the hardware store. If you are doing this, make sure you don’t have an extra o-ring residing in the groove inside the hydro-boost. Once I got rid of this extra o-ring and put a new one on the mating male end, it worked nicely.

Figure 5 This shows the pressure lines to the hydro-boost. Left one (as you look at it) goes to the steering pump. Right one goes to the steering box. O-rings are on the solid lines inside the hydro-boost. The return line is next to the right pressure line.

The Edelman P/N's for the two pressure hoses are:

71415 pump to hydro

71416 hydro to gear

This is from model year 1988 to model year 1993 F450

Next, I disconnected the old vacuum hose and removed it. Then I plugged the port on the firewall vacuum manifold with a rubber cap. If you don’t do this, your air vent doors won’t work in your cabin air system.

The new master cylinder had a switch used for disabling the cruise control during braking. I understand that not all the master cylinders have this switch. It got in the way of my air intake, so I removed it and put a ¼ inch inverted flare plug. Note that some of you may want to use your old master cylinder instead of the one that comes with the hydro-boost. Towcat says it’s possible, but the distance between centers of the mounting studs on the boost unit need to be the same as the holes on the old master cylinder. Mine didn’t fit, so I used the master cylinder that came with the hydro-boost.

Figure 6 Red connector is part of cruise Figure 7 Pressure line to steering pump

control disconnect switch which was showing the compression fitting joining

removed and plugged. the old end on the new line.

One difference between the two master cylinders is the fluid reservoir. On the F-450 unit, the tank is larger and both the front and rear braking systems use this same reservoir for fluid. On my F-350 master cylinder, there are two different reservoirs---each one feeding the front or rear set of brakes.

Next, I filled the steering pump reservoir, turned on the engine and rotated the steering wheel lock-to-lock a few times. Then I refilled the reservoir and took it out for a test run. After the run, I checked the fluid level again. I think air will be escaping from the system for a while, so I expect to check it again. Note that if your fluid level gets low, your steering pump will begin to make a lot of noise.

The brakes work well now, with a much shorter stopping distance and firmer pedal. My thanks to Towcat for helping me figure out how to do this. Total time, including parts chasing, was several days. It seemed like chasing parts was a bigger job than the installation. If you take on this project and think of something that needs to be added to this article, shoot me a private mail and I’ll update it.