View Full Version : Custom Brake Booster

09-08-2015, 10:49 PM
I've got a pair of Offenhauser valve covers that I've wanted to use on my 390, but my OE booster is about 1/2" too long. Not wanting to cut the cast covers, I'm looking to "shrink" the booster instead. Since the old booster is original, one of the only parts of this car that has not been replaced, it's now time for it to go.

I found a Tuff Stuff 2131 "slim line" that looks like it will fit. No one has dimension drawings for this or can tell me what the bolt pattern is, so I had it delivered to see if I can modify my existing bracket.

The MC looks smaller too, so that should eliminate the interference with my diagonal brace on that side.

09-08-2015, 10:53 PM
I measured up the parts as best as I could and it all looks like it will fit, so I went ahead and started modifying and fabricating.

Here's the old booster. There is 2-15/16" between the bolt flange and the center of the 3/8" diameter rod end. The aluminum cylinder protrudes exactly 2".

09-08-2015, 10:57 PM
The new cylinder is a larger diameter and some type of plastic. It fits inside the bracket but I had to cut it to length. I used a Dremel cut-off wheel and wore a paint respirator. Whatever that stuff is gives off smelly, irritating fumes. I used tape to make it then cut away.

09-08-2015, 10:59 PM
After cutting:

09-08-2015, 11:04 PM
Next was to cut the threaded rod to the right length. I'm using a spherical rod end to mimic the fitting on the bracket, which fits a 3/8" diameter rod. Measure both the rod and rod end. My rod end is deep to allow for adjustment; I had to cut it down to fit. As long as you have 3/8" depth of threads for a 3/8" rod you have full strength. Measure twice, cut once!

09-08-2015, 11:10 PM
Like I said, measure twice. Here I'm using the cut end of the (male) rod to check the available rod end (female) depth.

09-08-2015, 11:12 PM
I don't have room for a lock nut so I used a big ol' blob of RED lock-tite. AFTER measuring again, of course...

09-08-2015, 11:15 PM
Inside the cylinder is a two-stage air filter. The soft foam is pictured in the last post. The outer part is a dense felt, which I had to remove and enlarge the center to fit around the rod end. Then I threaded it on one final time over the red lock-tite.

09-08-2015, 11:16 PM
The rubber bellows didn't fit over the rod end, since this steel insert was inside it. I just removed the insert and stretched the bellows over the rod end.

09-08-2015, 11:18 PM
The bolt pattern on the new booster is slightly smaller (in both directions) than the OE bracket. To make it fit, I just enlarged the holes. Progress 1/32" at a time to keep the new hole centered at the original location. For this application, a 1/2" final hole size was perfect.

09-09-2015, 10:17 PM
Bolted up, everything fits. I have one, maybe two toothpicks clearance.

09-09-2015, 10:23 PM
As my last dual master modification, there is interference with the OE diagonal brace. Last time I modified the cylinder cover, which took a lot of time and looked terrible when I was done. Since I have a parts car available, I went ahead and modified the brace this time instead.

A little cutting, bending, and finally hammering a little bulge to just clear the edge of the cover.

09-10-2015, 09:23 AM
Holy shock towers, Steve... What happened to this one?

09-11-2015, 08:18 AM
That's just some red primer under the bracket that I removed for access.

09-11-2015, 11:16 AM
That's just some red primer under the bracket that I removed for access. http://www.squarebirds.org/vbulletin/attachment.php?attachmentid=11746&stc=1&d=1441891372You might consider welding a heavy plate on top, then buy new front shocks. Is the other shock tower the same?

I'm worried about the spring coming through... - Dave

09-11-2015, 11:53 AM
That's what the shock towers look like when you remove the heavy domed brackets. I don't see anything unusual.


09-11-2015, 12:10 PM
Ahh, there they are... The shock looks like the nut on top is frozen to the threads and without the brackets the whole tower looks mighty weak. This holds up half the front end as the upper spring perch is just underneath the top of the tower.

09-11-2015, 04:33 PM
You might consider welding a heavy plate on top, then buy new front shocks. Is the other shock tower the same?

I'm worried about the spring coming through... - Dave

That's all nice solid metal, painted with red primer. The other side has the bracket and cap installed. Those pieces hold the shock in place.

09-11-2015, 07:08 PM
Thanks Steve and John, you've taught me something I didn't know.

Now, I got it filed in the forefront of my brain cells. Gee, I hope I can remember this six months from now... Thanks again. - Dave

09-11-2015, 07:17 PM
Thanks Steve and John, you've taught me something I didn't know.

Now, I got it filed in the forefront of my brain cells. Gee, I hope I can remember this six months from now... Thanks again. - Dave

Six months? Six days is more like it with me.


09-11-2015, 10:45 PM
Thanks Steve and John, you've taught me something I didn't know.

Now, I got it filed in the forefront of my brain cells. Gee, I hope I can remember this six months from now... Thanks again. - DaveWhat year is your Bird? I thought 58-66 were the same chassis.

09-14-2015, 11:36 PM
Here we are with final assembly. For the plumbing, I had to drive to King, NC to a hydraulic hose maker and get a special fitting made so my front brake distribution block would fit between the new "fattie" master cylinder and the hood hinge.

09-14-2015, 11:39 PM
And an overall view.

01-16-2016, 08:54 PM
wow hey thanks for the reply back and great information.

09-26-2016, 03:31 PM
Did you use the original vacuum booster linkage for your installation?

09-28-2016, 03:40 PM
Did you use the original vacuum booster linkage for your installation?

Yes. See post 10.

05-02-2017, 09:27 AM
Not satisfied with the overall braking performance, I've decided to take Dave Dare's advice and install a combination valve. Summit racing has a nice one for the same year Chevy sedan; the weight bias is about the same. They also sell stainless steel line and a neat flare nut set that fits all the "odd" size ports in the setup.

Combination valve:

Flare nuts:

In order to fit the installation between the master and hood hinge I'm using two-inch diameter loops to go up then down into the top of the valve.

The stainless is not easy to double flare and bend as cunifer, but it looks terrific. Some patience and care, oiling the tool when flaring gives good results.

05-02-2017, 05:24 PM
Ever since OEMs came out with disk/drum brakes they have also included a proportioning valve (because it is necessary). I'd love to take credit for the idea but sadly, I can't.

I often reference Marcelo's brake job (because I don't have a Squarebird but he does):


Marcelo didn't use an expensive bending machine, he used nothing more than a pair of Harbor Freight brake pliers for all these bends. Notice how close the bend is to the nut on the brake switch block. Notice there are no loops and how 'clean' this job looks. Conventional bending tools cannot bend this tight so coils of tubing are required. Not any more. Simply slide the nut on tubing, flare the end, then position the pliers right up to the back of the nut and bend the tube. It's easier to do than to talk about it.

If space is an issue, use the right size nuts instead of buying expensive adapters. Coils produce much more resistance, they flex under pressure and they eat up more tubing. Think of one coil loop as being four ninety degree bends.

Carbon steel line, stainless line OR conifer brake line bend equally as well in bending pliers. I've never kinked a line using the pliers and it's the only bending tool needed for the whole job. For practice bending, I use one of the old lines I'm replacing. I also use old line for bench bleeding my new master cylinder; simply screw the nuts into the M/C ports and bend the old steel line up and over the reservoir but make sure the ends are below the surface of the brake fluid. Bending pliers make this silly-easy, even on steel line. HF wants about $12 for a pair of bending pliers. (http://www.harborfreight.com/tube-bending-pliers-95782.html)
- Dave

05-03-2017, 09:06 AM
Dave, any thoughts on which ports on the master to feed which half of the car? I've noticed that old Fords that I've owned use the front bowl for the rear, and the rear bowl for the front. In a master with two large bowls I don't see why it would matter.

Also, it appears in your picture that the brake light switch is on a front port. Since the combination valve applies pressure to the rear brakes first, the switch should be on the rear half of the system.

05-03-2017, 11:29 AM
I'll say the rear reservoir is used for the front brakes on Ford and Mopar. GM uses the opposite assuming both reservoirs are the same size. This is according to a company named Stainless Steel Braking Corporation and found in one of their proportioning valve installation instructions.


05-03-2017, 03:48 PM
'Rule of thumb' is... The larger reservoir feeds disk brakes (because they have 2.5" (63mm) pistons/each).

Since dual master cylinders have the same diameter pistons (yes, there are two separate pistons but on the same shaft), if your reservoirs are the same size, it doesn't matter which one services the front.

For the same piston diameter reason, your brake light switch can be plumbed to either front or rear systems. It doesn't matter which one you use.

Truth of the matter is... Your combination valve FRONT ports, all three, are open to the M/C. (Check this out by blowing on the top front port as you feel both front exit ports.)

So, where does the metering come in for the rear system? Those huge front pistons take a lot of pressure to 'pinch the rotor' whilst the rear system has small pistons that very easy expands shoes.

So... you apply hydraulic pressure, both circuits get charged but the rear doesn't 'hold back' right away, causing the rear circuit to 'grab' and stop first. As more pressure is applied, a spring in the rear circuit is overcome and the valve starts proportioning which 'holds back' the rear circuit as the front pistons always advance with full pressure.

I hope this explanation helps. - Dave

05-03-2017, 09:02 PM
I'm not a brake expert but my understanding of how a dual master cylinder works is that. When you press the brake pedal that activates the rear brakes first. Once the shoes make contact with the drums that then activates the front brakes. This obviously all happens within seconds. The proportioning valve as Dave said Is there to regulate the pressure between front and rear to avoid rear brake lock up. As far as the switch goes I would not think it would make much of a difference as to wether it is on front or back.

05-03-2017, 09:23 PM
Phil, some of the OEM master cylinders had a valve in them for the rear but it was not a proportioning valve. They are not 'time-based' but rather, 'pressure-based'. Just like your accelerator pedal, you don't always mash the brake pedal down all the way, but you regulate your machine according to your needs. Aftermarket masters don't come with any valves.

Even so, OEM's still used proportioning valves if they had a disk/drum setup (which most of them did).

A combination valve does MUCH more. It:
Meters to the rear first,
Proportions the pressure (not the flow) between front/rear systems,
It has a 'center spool' that shifts between both systems, tying the hydraulics together, mechanically. <--this is important.

It also shuts off either the front or rear system in case a line ruptures or a wheel cylinder lets loose. When this happens, the electrical terminal grounds an "emergency ' light.

The system is good, and very reliable. I have it on two classics. - Dave