I presume you are saying that 25 inches of vac is not providing adequate braking. You don't mention what year your car is but if it's a 65-66 that stock booster is pretty puny, I think, designed for power drum brakes which require about 50% of the pressure needed by those big discs.  I'm running the 12.5 inch GT brakes with Explorer discs in the rear and with only 9-10 inches of vacuum it had terrible brakes.  I have the MustangSteve power brake pedal conversion and had a 9" Fox 4-Cyl booster.  I changed to the Fox V8, 7" dual diaphram booster and added a mechanical vacuum pump which now provides me with a solid 17" of vacuum.  Great brakes now. 

BB1

I installed a hydroboost system a couple of years ago and am quite pleased with it.  It was "tight" getting everything to fit on the firewall as I also have hydraulic clutch.  Of course the 71-73 Mustangs are wider than a 65/66, so that might not be an issue for you.
"Hydrotech", based in Tennessee, has kits and was very helpful with technical issues.
I had to change to an offset master cylinder for the clutch, available from "Modern Driveline" and a brake MC for a 95-97 Explorer, Raybestos #MC390269.
Let me know if you want more details.

I put hydroboost in my 62 Galaxie and LOVE it.  here is some info on my install

Daze on galaxieforum.com wrote:

I was PMed about my hydroboost in my Galaxie and figured I would make a post so everyone could benefit.  Below are several reposts of the project:

on the old forum, daze wrote:

The rear drum brakes on my galaxie are locking up way to soon despite the aftermarket proportioning valve installed in line.  Because of this I need to do some rear brake work. I don't see the point in investing in drums especially when I have a disc brake set up  sitting on my parts shelf. Of course if I go from disc/drum to disc/disc the MC will need to be changed out. The MC I already have on the shelf for this project is a corvette unit, so that left me with two options modify the mounting holes on the corvette MC to match the Fordpattern, OR install the hydroboost unit I also have had sitting on the shelf and have planned on putting on the galaxie any way. The corvette MC will bolt directly to the hydroboost unit without modifications and special mods had to be done to mount the hydroboost to the car anyway so why modify the MC now and then they hydroboost unit mounting plate later when I can just install the hydroboost now and only modify once.

Since my Galaxie is still drivable I am getting all the parts together and ready to install first, that way the amount of time the car will be down will be minimal. I started by working out the mounting between the hydroboost and pedal assembly. I had an extra pedal assembly lying around so I had something to work with. First thing to overcome was the length of the pushrod. on my hydroboost unit the pushrod is an integrated part of the unit and was to long to
allow me to bolt the hydroboost directly to the fire wall and still hook up to the pedal. One possible solution was to cut the the rod down to size thread it and then thread on a shorter rod end. I didn't like this idea for a couple of reasons first the rod has to be narrow enough to slide inside the hydroboost unit so I couldn't cut off that much AND what would I do if I had to replace the unit down the road, modify another??  A better solution seemed to be to build a stand off 

Why hydroboost you may be thinking?? well:
1. the hydraulic clutch MC on my car would have clearance issues with a normal vacuum booster unit.
2. hydroboost does not draw on engine vacuum so at idle the power brakes would be just as good as at higher RPMS.
3. hydroboost provides more brake pressure than a vacuum booster
4. unlike with vacuum power assist if the power part of the brakes were to stop due to the engine shutting off, there is no difference between
manual brake pedal effort and pedal effort (assuming the same bore MC) when going through a non functional hydroboost unit. 

daze wrote:

WOW!!!  I love my new hydroboost power brakes.  If you want to stop, you stop right now!! 
 
My set up consists of a mid 90s Chevy S10 hydroboost, 67 corvette master cylinder and home made lines using ends and lines from aeroquip.  The power steering pump is a stock unit.  I have T-bird front discs that I have had on the car for a couple of years and MarkVII rear discs that were installed as part of this project. 
 
The project started by working on getting the hydroboost unit to be correctly spaced on the fire wall and still maintain the correct pedal height with out having to modify the hydroboost push rod.  I happened to have an extra Galaxie pedal support and pedal and bolted it to my hydroboost unit to mock things up. 
 

 
Once the hydroboost unit was ready to go in I turned my attention to the car and began installing the Mark VII rear discs. 
 

 

 

 

 
With the rear discs installed I removed the old Ford master cylinder, then mounted the corvette master and the hydroboost unit in place and plumbed the brake lines. 
 

 

 

 
I then added a small oil cooler to be used to keep the power steering fluid cool. 
 

 
 
 

 
with all the pieces in place I ran the high pressure power steering lines and started on the low pressure lines.  The original low pressure line from the power steering unit went to a T.  The low pressure line from the hydroboust unit also went to that T and then a single line went out from the T to the oil cooler and then back to the return port of the power steering pump. 
 
With it all hooked up, I primed and bleed the power steering system, and then tested out my new power brakes.  The results were less than spectacular.  When you first fired up the car the brake pedal would drop about 1/4" creating a slight brake drag.  as soon as the car was turned off the pedal would come back up to normal height.  Also the brakes were ultra sensitive.   
 
I had purchased some fitting for this project from hydratech so I contacted them and told them what parts I had in my system.  Shortly after I sent the email I heard back from Paul at hydratech.  He was extremely helpful and new exactly what my problem was and what the potential fixes were.  On a hydroboost unit too much back pressure on the return line will cause the pedal to self apply.  He gave me a quick test for confirming that back pressure was indeed the issue and a list of 4 possible causes of the extra back pressure.  Turns out I had all 4 and even when eliminated I still had to much back pressure.  The solution was to ditch the T and add a second return line port to my power steering reservoir.  To do so I took a 3/8" bolt and machined it to be a thread in nipple. 
 
 
 

 
I then drilled and taped the reservoir to accept said nipple 
 
 
 
I chose to tap the hole in the reservoir for two reasons.  I wanted to be able to thread it in from inside and I wanted to be able to use RTV as a thread sealer to avoid any leaks.   
 

 
From there a nut and some more RTV was applied to the outside and tightened down. 
 

 
When I reinstalled the reservoir, plumbed it all back together and fired it up it performed flawlessly.  In fact I am so impressed I would recommend a hydroboost unit over a vacuum assist to any one who has power steering. 
 

 

older post on this forum Daze wrote:

When I set it up originally I used a Corvette Disc/Disc power MC.  It worked well, I could stop on a dime but there was one small flaw.  The top of the pedal pressure felt odd, not spongy like there was air in the system... but odd.  It was almost like the brakes were slow to grab then after the pedal was pushed a little further they would grab no problem.  The other issue I was having is if I got on the brakes hard (simulated panic stop) some fluid would push its way out of the top of the MC cover.  to solve these issues I decided to get the MC from the same make and model that the hydroboost came from (94 Chevy C1500)  It was a quick swap this morning, a little reverse bleeding and I was off to a test drive.  I couldn't be happier with the results.  the brakes were really good before but now they are 10 times better!!!!!  The new MC is not as flashy as the corvette one, but I like it that way.  It tones down the whole system and makes it blend in to the engine bay.

Before

After


 

Since I originally put this info together I have made a few more changes.  I ditched the oil cooler for a transmission cooler and mounted it in front of the radiator.  The transmission cooler is less restrictive and a bigger surface aria so it cools better. Doing this also helped my PS pump to run quieter.   I also added a GM J-car type R&P to my Gal which required me to use a GM type II power steering pump.  This had the benefit of a further reduction in noise and had no effect on breaking positive or negative.
 

Daze great write up thanks :-) And thanks everybody else for hints and help. If I put this in it will be a Winter Project. But there is a lot work going into the pipes/hoses.

regards Rob

Dkralc wrote:

I put Hydro-boost is my 66 Mustang and loved it.  Here is a write up it did at the time https://fyi.boardhost.com/viewtopic.php?id=6166.

Also have Hydro-boost in my Factory 5 roadster stops on a dime.

Great write up thanks :-)

Nos681 wrote:

It 
Is your booster leaking and what size master cylinder are you using?

Booster is new no leaks, master cylinder is 1 inch bore for disc/disc. bandit vacuum pump 25 in/hg

351c only produces about 10 in/hg at idle, healthy cam :-)

TKOPerformance wrote:

Have you pulled a vacuum on the booster and let it sit overnight to see if it held?  I've seen new/rebuilt ones that had a tiny internal leak that would cause problems like what you describe. 

Its connected directly to the bandit pump. The pump has a pressure regulator that starts if the vacuum is to low. the car can sitt for Days and I still have 3 brakes pushes Before the pump starts. So from that I say yes it holds the vacuum good.
 

TKOPerformance wrote:

Is it still connected to manifold vacuum at all? 

No goes directly from the bandit pump to the brake booster. Breakes are good but I belive that the hydroboost would take it to Another level.
 

50vert wrote:

I can't see the hydraboost doing anymore than what you have now. If the engine stops, you won't have any boost at all.

That is not how hydroboost works.  Hydroboost has an internal pressure canister that if the engine stops will provide assistance for one stroke of the pedal.  After that one stroke its just like vacuum assist, it still works however it's stiff.
 

Just a tip.

If using a Ford p/s pump, there is a cover that has 2 return lines for such an application.
Such as a 1992 F350 with a diesel.

TKOPerformance wrote:

I guess it depends on if the pump can keep up with demand.  Are the brakes the same if you have to apply them in rapid succession?  In other words, is the first stop the same as the second, third, etc. in a short period of time? 
 

yes all the stops are the same, it has an built reservoir that handles 3 stopp before the pump starts.