HydroboostSwap

HYDROBOOST SWAP

A hydroboost is a brake booster that uses hydraulic pressure instead of vacuum like a normal brake booster. They have been in use since the 1970's, but usually only in heavy duty applications, such as large trucks, or where a vacuum booster wouldn't fit, like in newer Cobra Mustangs. Although they all look alike from the outside, they have different internal sizes. I have read the Cobra is the smallest one, and 1 ton trucks have the largest. All the newer ones have a pressure reservoir so that it still works if the engine quits.

I was not too happy with the brakes in my Mustang. They were okay, but I could not get them to lock up no matter how hard I pushed. In the wet the rears locked up before the front simply because I couldn't generate enough pressure. So I was thinking about changing the booster or altering the brake pedal ratios.

When I was doing my Saginaw Pump Swap I noticed that some Astro vans had hydroboost and some didn't. [ I don't know why. They all seemed to have the same brakes, engines, etc. ]
There was one van with the hydroboost removed, so I peeled off the gasket and went to find a Fox body Mustang without its' booster. I was surprised to find that 3 bolts and the center hole lined up perfectly. After some thought I came back and removed the unit and lines and paid $20 for it all. [I think the astro spacing is called a 'standard bendix pattern', which is why both Fords and GMs have it.]

Here is a cutaway drawing of the Hydroboost unit.

The input rod is pushed by the brake pedal, which pushes on the boost piston and then the output rod.
Even without any hydraulic pressure you still have brakes.

When the input rod moves forward the lever moves the spool valve which closes the bypass and lets pressure into the boost pressure chamber. A light touch gives some pressure, while a hard stomp gives full boost.
Hydraulic pressure pushes on the rear of the boost piston which helps you push on the brakes.
A bigger piston or higher pressure gives more boost.

The accumulator holds pressure when the engine is off.

Simple isn't it?

Here is the Hydroboost cleaned up and painted. Below it is the brake pedal rod end from a mustang like mine. Some are curved downward and others are straight.

It was necessary to cut off the Astro van rod and weld on the mustang piece to get the right pedal height. This would prevent binding and allow the brake light switch to work without modification.
I drilled out the center so it would fit over the stock rod.

As luck would have it the photo shows it in just about the correct length. [4 7/8" from the firewall to the center of the rod end.]

This is the rear shot after the mods were done. One bolt was moved and the brake rod is welded on.

Cutting off the corner of the bracket is not required for clearance. It was just an easy way to get the bolt out. The bolts are 10mm versus 3/8 stock. This meant the holes in the firewall had to be enlarged very slightly.

The rear of the Hydro has a giant nut holding it on. Luckily it didn't need to be removed in this installation.

The stock rubber fits on the hydroboost no problem.

These are 2 brake pushrods. the top one is from the hydroboost and the lower was pulled out of a Ford with vice grips. The Ford has an adjustable tip.

The ford piece was shortened, beveled, and welded to the other one while held in the lathe.

Another great photo.
I wanted to keep the Ford master cylinder but it has a different bolt pattern, so the holes were redrilled using a jig made from another master cylinder. it's tough drilling half in steel, half in cast iron. Do they make cast iron bolts?
An adapter ring was also made to fill the center hole.

The pushrod is also installed. It kept falling out, so I took apart a few hydroboosts at the wreckers. There is a plastic disc that snaps around the pushrod and mine was missing. [I thought a hundred parts would come flying out but it's pretty simple inside. The brake rod pushes a spool valve which closes the bypass and so the power piston pushes on the master cylinder. A ford car had a smaller piston than the astro van.] When I took apart mine the plastic disc fell out from somewhere inside.

A hydroboost can be installed any way you want. There is no 'up'. I wanted to put the hoses downward for neatness.

The way I wanted to install it the pressure reservoir cylinder hit the inner fender. An 'H' cut was made and the center parts were pushed out until the hydroboost cleared. Then the outer pieces were hammered in and welded. Scrap metal filled the hole, and it was welded from both sides and painted. At least there's room for a big vacuum booster if this didn't work.

I can see why Cobra units are installed with the reservoir up. To the inside it would hit the head, and the outside hits the fender. So they installed it so the master cylinder flange is sideways. So why not cast a new nosepiece? Because the spool cover is in the way. Easier to cast a new master cylinder with a vertical flange.

When I began test fitting the unit I realized that the master cylinder would now be hitting the proportioning valve. This seemed strange until I realized the stock booster angled up while the hydroboost came straight out.

Something had to be altered, so the adapter plate had the center cut out and rewelded to match the stock angle. I was able to do it without removing the hydro, but it ain't too pretty.

Carpet foam was glued to the back for sound deadening after the edges were painted black.

Here is the bottom showing the lines. It is very tight under there so all three were arranged parallel beside the frame rail. The line from the pump is brazed from 3 pieces that were sleeved together inside. I think the astro van rubber hose was used.

The stock steel line goes to the steering rack line, which is removed from the PS pump.

The return bypass to the power steering reservoir is altered from the straight stock piece. It was retapped for a standard pipe nipple. The hole was filled with cotton batting first and dug out after, to stop shavings from getting inside. [and then I took apart the unit later anyway to replace the pushrod clip.]

I just brazed a new end on the steering rack hose a few weeks before for my PS project. Now it was cut off and brazed to the steel line from the hydro. Since the Hydro was at an angle now a bend was put into it to adjust it's path.
The stock Mustang PS line is long enough to reach the firewall!

The 2 pressure lines go under the clutch cable bracket, while the bypass goes above. It blocks access to the prop valve a bit.

To connnect the line to the pump I found a tube with a nice curve in it. This would give me more room for the oil filter removal.

Finished installation. I filled the pump with fluid, started the engine, and refilled it a few times.

My first drive was disappointing. Sometimes it would be great, but other times there was almost no brakes. The power steering was squawking again, and the pedal was very slow to return. After a few days it was better but not great.

I found a webpage called http://www.hydratechbraking.com/, and it had some tips. Firstly, the hydroboost takes some time to 'de-gas' itself from thousands of tiny air bubbles. These bubbles make it act weird. Secondly, use only real PS fluid, not auto tranny fluid. And thirdly, the PS pump has to provide adequate flow and pressure.

I was still using the stock Ford valve in the saginaw pump, so I went and got a valve from an astro van and a huge old Ford car with hydroboost. The car had a valve with one washer, and I tried that. After letting it de-gas again it was better, but a few days later I removed the valve and took off the washer for higher pressures.
I also drilled out the pump output fitting to 9/64 to increase flow. These changes gave me the brakes I wanted.

The last time I filled the reservoir I used Penzoil PS fluid, which is blue. It de-gassed much faster than clear fluids.
[ I read on the Net that these blue fluids for PS or Auto trannies are just farm tractor hydrostatic fluid. Whatever it is, it works nice.]

Overall it was a success. I can lock up the brakes and they seem more balanced. It's amazing that the brakes stop so much better with the same discs, rotors, pads, and master cylinder. At first I couldn't stop being an ass and hammering on the brakes hard. [ Sorry about your dentures grandma. I think they went down the heater vent.]

I was worried that the steering would be overboosted with the higher pressure but it didn't have much effect.

I don't think the pressure reservoir is holding fluid properly. But what do you want for 20 bucks? [ It started to work better after a few weeks. perhaps it was gunked up]

Using hindsight, I could have installed the hydroboost with the master cylinder flange vertical, then made a diamond shaped plate adapter to bolt to the hydro with 2 vertical holes, with a central hole and 2 horizontal bolts to hold a Ford master cylinder. It would have saved me re-drilling the hydro or cutting the fender.

June 2006

Update March 2007

I finally found some of the additive recommended for hydroboost and power steering in general, called LubeGard PS Fluid Protectant. [also converts ordinary PS to Honda PS fluid. snazzy.]

After I drained the old stuff I decided to check something that had bothered me. Once I had added the hydroboost the steering had become less 'smooth'. The original return fitting had a teensy hole in it, perhaps 1/64 or smaller, while the new 90 degree fitting I added had a 5/16 inch passage. Much Bigger. Perhaps this let too much fluid bypass the steering.
So I made a restrictor from brass that has a 3/32 hole. This seems to have made the steering smoother and the brakes a little bit more powerful. Next time I'll make it even smaller.
The additive did work well and degassed the new fluid very quickly.

A week later I found a brass tube 3/32 outside and 1/16 inside. I removed the return hose, pulled the fitting and soldered the tube in. Only a dribble of fluid came out.
No great difference, but the brake pedal was a bit slower to come back up.

	Here is the Hydroboost cleaned up and painted. Below it is the brake pedal rod end from a mustang like mine. Some are curved downward and others are straight. It was necessary to cut off the Astro van rod and weld on the mustang piece to get the right pedal height. This would prevent binding and allow the brake light switch to work without modification. I drilled out the center so it would fit over the stock rod. As luck would have it the photo shows it in just about the correct length. [4 7/8" from the firewall to the center of the rod end.]
	This is the rear shot after the mods were done. One bolt was moved and the brake rod is welded on. Cutting off the corner of the bracket is not required for clearance. It was just an easy way to get the bolt out. The bolts are 10mm versus 3/8 stock. This meant the holes in the firewall had to be enlarged very slightly. The rear of the Hydro has a giant nut holding it on. Luckily it didn't need to be removed in this installation. The stock rubber fits on the hydroboost no problem.
	These are 2 brake pushrods. the top one is from the hydroboost and the lower was pulled out of a Ford with vice grips. The Ford has an adjustable tip. The ford piece was shortened, beveled, and welded to the other one while held in the lathe.
	Another great photo. I wanted to keep the Ford master cylinder but it has a different bolt pattern, so the holes were redrilled using a jig made from another master cylinder. it's tough drilling half in steel, half in cast iron. Do they make cast iron bolts? An adapter ring was also made to fill the center hole. The pushrod is also installed. It kept falling out, so I took apart a few hydroboosts at the wreckers. There is a plastic disc that snaps around the pushrod and mine was missing. [I thought a hundred parts would come flying out but it's pretty simple inside. The brake rod pushes a spool valve which closes the bypass and so the power piston pushes on the master cylinder. A ford car had a smaller piston than the astro van.] When I took apart mine the plastic disc fell out from somewhere inside.
	A hydroboost can be installed any way you want. There is no 'up'. I wanted to put the hoses downward for neatness. The way I wanted to install it the pressure reservoir cylinder hit the inner fender. An 'H' cut was made and the center parts were pushed out until the hydroboost cleared. Then the outer pieces were hammered in and welded. Scrap metal filled the hole, and it was welded from both sides and painted. At least there's room for a big vacuum booster if this didn't work. I can see why Cobra units are installed with the reservoir up. To the inside it would hit the head, and the outside hits the fender. So they installed it so the master cylinder flange is sideways. So why not cast a new nosepiece? Because the spool cover is in the way. Easier to cast a new master cylinder with a vertical flange.
	When I began test fitting the unit I realized that the master cylinder would now be hitting the proportioning valve. This seemed strange until I realized the stock booster angled up while the hydroboost came straight out. Something had to be altered, so the adapter plate had the center cut out and rewelded to match the stock angle. I was able to do it without removing the hydro, but it ain't too pretty. Carpet foam was glued to the back for sound deadening after the edges were painted black.
	Here is the bottom showing the lines. It is very tight under there so all three were arranged parallel beside the frame rail. The line from the pump is brazed from 3 pieces that were sleeved together inside. I think the astro van rubber hose was used. The stock steel line goes to the steering rack line, which is removed from the PS pump. The return bypass to the power steering reservoir is altered from the straight stock piece. It was retapped for a standard pipe nipple. The hole was filled with cotton batting first and dug out after, to stop shavings from getting inside. [and then I took apart the unit later anyway to replace the pushrod clip.]
	I just brazed a new end on the steering rack hose a few weeks before for my PS project. Now it was cut off and brazed to the steel line from the hydro. Since the Hydro was at an angle now a bend was put into it to adjust it's path. The stock Mustang PS line is long enough to reach the firewall!
	The 2 pressure lines go under the clutch cable bracket, while the bypass goes above. It blocks access to the prop valve a bit. To connnect the line to the pump I found a tube with a nice curve in it. This would give me more room for the oil filter removal.
	Finished installation. I filled the pump with fluid, started the engine, and refilled it a few times. My first drive was disappointing. Sometimes it would be great, but other times there was almost no brakes. The power steering was squawking again, and the pedal was very slow to return. After a few days it was better but not great. I found a webpage called http://www.hydratechbraking.com/, and it had some tips. Firstly, the hydroboost takes some time to 'de-gas' itself from thousands of tiny air bubbles. These bubbles make it act weird. Secondly, use only real PS fluid, not auto tranny fluid. And thirdly, the PS pump has to provide adequate flow and pressure. I was still using the stock Ford valve in the saginaw pump, so I went and got a valve from an astro van and a huge old Ford car with hydroboost. The car had a valve with one washer, and I tried that. After letting it de-gas again it was better, but a few days later I removed the valve and took off the washer for higher pressures. I also drilled out the pump output fitting to 9/64 to increase flow. These changes gave me the brakes I wanted. The last time I filled the reservoir I used Penzoil PS fluid, which is blue. It de-gassed much faster than clear fluids. [ I read on the Net that these blue fluids for PS or Auto trannies are just farm tractor hydrostatic fluid. Whatever it is, it works nice.]