Customs dual master power booster upgrade not working

to find the ratio at the booster you need to find the pedal ratio to the link A/B then need to find the link ratio Al/Bl ( the bell crank ) then its A over B times Al over Bl = the overall ratio basically its the pedal ratio x the bellcrank ratio = overall ratio



page 71 if anyone has Puhns book as I cannot scan it to the computer

 

I drove the car a few miles after the shop finished what they had to do and I really felt like there wasn't any power assist. Had to push down on a hard pedal to get it to stop - basically like manual brakes. I drove it to the Classic Car Restoration garage that did the original work on my car and the shop owner test drove my car and said the same thing.

I took measurements per the specs. A=12" B=3" C=1 1/2" and D=2 1/2". That gave me a calculation of 2.4:1 ratio if I did that correctly, but I wanted to mention a few things that we tried first to see if you have any thoughts...

1) First thing I noted was that I was not getting brake lights when the engine was off and I pressed my foot down at a light pressure on the brake pedal. Originally with the Treadalvac single master/power system the brake switch was mounted on a port on the master and I easily got lights. The same switch assembly is now mounted in an upright T that joins the rear brake line to the front port on the new dual master. When we start the engine and get vacuum and the pedal gets a little easier to push, I get lights. To me, that seems like there's not enough pressure being generated when the car is off to engage the pressure light switch. I'm wondering if this could signify another problem?

2) At the request of the manufacturer, we removed the booster from the car that the shop installed and tested it with vacuum and tried manually pushing the rod through and it was extremely hard to budge, so we determined it was defective. We put a replacement booster that we pre-tested on the car and without the master cylinder attached and the engine on, providing vacuum, I pushed down on the stock pedal and it felt as I'd imagine it should. The rod pushed through freely without any terrible effort. As soon as we re-attached the master cylinder, I got about an inch of travel and then a hard pedal again.

We've been racking our brains because it seems like it has to be some sort of pressure issue somewhere from the master cylinder on and not from the booster back. I don't know if that would translate to us having too MUCH pressure or too little? Seems to be too much or we'd have a soft pedal. I don't know what would cause that issue, though, because the brake lines up to the T, which worked with the Treadalvac, are all the same. The only new lines are the ones coming up from the Ts going to the Master. They are a wider diameter than the lines that are on the frame. Those are 3/16, I believe, and the others might be 1/4". But it's wider to smaller, so that shouldn't cause less pressure.

Any thoughts? I'm lost right now and the garage is wondering if a 7/8 bore master would work better, but I feel like that would cause more pressure and a worse problem...I'd imagine if there was a real problem with the pedal, then it should be consistent and not change when the Master is installed. Also, when I say 'hard pedal' I mean it becomes very stiff and takes more than normal pressure on it to get it to stop the car, and when the master cylinder was off with the engine on it felt 'normal.'

 

If your calculations are correct, and the pedal ratio is indeed 2.4:1, then that pedal assembly has to go.

You should have 5:1 - 6:1.

You will never get good braking pressure with 2.4:1

 

What gipsy said. You can think about it as trying to remove your lug nuts with a stubby ratchet , it just isn't enough leverage to be effective. You need that pedal ratio to get the leverage to ease the effort



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Yup, as they said. Think, too, that a 7/8 bore will act like a bit more leverage, increasing the pressure to the cylinders, while Decreasing the pedal effort, which is what you WANT!! That 1 inch master is too large, which is why you have a hard pedal with little travel. You are thinking this brake leverage thing backwards: smaller bore master equals less pedal pressure for same pressure in lines; larger bore equals more pedal pressure for same pressure in lines.

You have no brake lights because of the air in the line going to the switch. If you do not change that, you will forever have that problem; it won't go away, ever. The reason it worked as stock is because you weren't trying to pressurise a three inch column of air. Air does not pressure actuate anything very well, and certainly not a hydraulic (i.e.: fluid) switch.

And I will, once again, argue for a booster from a FULL SIZE car.

Cosmo

 

I would put pressure gauges in the system to see what you actually have. You would be able to see if the booster increases the pressure. A 7/8 inch bore master should increase the system pressure, decrease pedal effort and increase pedal travel.

 

Fix the pedal ratio and you will be fine! Gary

 

Fixing the pedal ratio using any of the existing components may be a lost cause.

I suggest a new pedal assembly. This will take some modification to the pedal assembly, as the firewall protrudes where the booster is. The bracket would need to be extended.

I have used numerous assemblies from Kugel Komponents, for the weirdest and most stubborn applications: http://www.kugelkomponents.com/zencart/index.php?main_page=index&cPath=4_7


Like this, but you'd have to trim the face to fit the protrusion, and get the pedal pivot farther from the firewall.

Or an adjustable one, like this:

http://www.dirttrackparts.com/product.cfm?InvKey=59421
Or

http://www.speedwaymotors.com/Universal-Under-Dash-Brake-Pedal-Mount,59785.html

 

Problem is I can't seem to find anyone around here that knows how to fix the pedal ratio. Unless I can find someone with a similar pedal system for a similar vehicle that we can replicate, I'm pretty sure nobody around here is going to fabricate one for me. They don't even understand the ratio requirements. I'm in the Albany, NY area if anyone has any suggestions.

 

The garage was looking at the bottom two but the way the inside of the firewall curves and the amount of space there didn't seem to fit. They are a bit too long to fit in that tight space. These are about 14" long and there's maybe 7" space without obstruction. the top one looks possible but might need some spacers to get it sitting flush. And that would mount it to the firewall. The booster is attached to the plenum. There is a gap now and always had a gap, but the brake assembly was centered to the booster rod, so I didn't think that was an issue. They don't have to actually be physically attached to each other correct? Wish one of you guys were closer to make this work.

 

I'm looking into that booster for a future conversion in the spring when I go to all Disc brakes. For now, I'd like to try to make this darned thing work. If you think the 7/8 bore master will do better, I will switch to that, just having a hard time finding one. As for the switch, I'd like to pull that dang thing out altogether. There's a guy with a 57 Buick Super, which is a larger car, same engine and treadalvac originally and he somehow made everything work with a 1" master and basically same setup, stock pedal and he said it works like a charm. Can't figure how that happened?

 

Look at the bottom two I posted (they look identical, except for finish).

They attach to the firewall via the bolts that currently attach your booster. You unbolt the booster and co-bolt the pedal bracket with it, and the firewall.

The tab at the far end gets attached to the dashboard, for stiffness.

The Speedway one is like $90. It may need to be modded a bit, but it WILL fix your problem, provided the pedal is an acceptable distance from the floor.

 

The booster is attached to a plenum which protrudes from the firewall by about 5" in the engine compartment, so the studs coming off the booster basically end up floating in the air. The firewall is solid except for the 2" hole that the piston comes out of, so the brake pedal can't mount directly to the booster unless the plenum is discarded. Here is a side profile illustration of the original setup...

 

Is the plenum welded on, or can it be, temporarily, unbolted?

 

I don't think the hydraulic system is the problem. It should work as connected.

I suspect that the pushrod between the brake booster and master cylinder may be too short. But more important for a big car that chintzy booster won't get it done. Read the Jeg's instructions and you can see the issues. Could be the pedal ratio but I've done plenty of these and never needed anything other possibly moving the factory clevis up.

http://www.jegs.com/installationinstructions/500/555/555-631010-116.pdf

 

PS. The old treadle-vac you removed had way more boost than the China repop you have.

 

After ditching the entire original pedal assembly, here's what I would do:

1. Obtain on of the pedal assemblies pictured above, probably the lowest one.
2. Verify that the bolt pattern matches the existing booster brackets. Re-drill as necessary.
3. Make a template of the studs holding the booster on the plenum.
4. Transfer that to the firewall, and drill holes in the same pattern.
5. Obtain some heavy wall tube with the I.D. of the studs that hold the booster on.
6. Measure the distance between the plenum and the firewall. Cut four lengths of heavy wall tube to that length.
7. Obtain Grade-8 hardware that is long enough to go through the booster brackets, plenum wall, heavy wall tube, firewall, and the pedal bracket, with room enough for nuts and washers.
8. Remove the booster studs from the plenum. Carefully grind them flush, and drill them out.
9. Use a step bit to enlarge those new holes large enough that the heavy wall tube will bass through.
10. Put the long bolts in the pedal assembly.
11. Mount the pedal assembly by passing the long bolts through the firewall and plenum.
12. Slip the heavy wall tube over the bolts.
13. Remount the booster and master cylinder.
14. Fit the pedal.
15. Fit-up the push rod, as necessary, to meet the booster.
16. Fashion a way to attach the end of the pedal bracket to attach to the dashboard.

This will stiffen the entire operation, attaching the booster directly to the pedal assembly, instead of floating sheet metal.

 

Tubing: http://www.onlinemetals.com/merchant.cfm?id=283&step=2&top_cat=197

 

Your issue most likely has to do with the much larger bore of the new master cylinder. When I converted my car (60 continental) to dual master I had to search far and wide to find a bore that resembled the original treadle vac. I ended up with the master out of an 87 Buick wagon. Which was a three-quarter bore, but I also bought the optional 5/8 bore for the same car. With the three-quarter bore the pedal is hard and the brakes work fine but I suspect it will be a much softer pedal with the 5/8 bore.

If you sit down and do the math on the pedal ratio and the bore size to figure out pressure at the wheel cylinders you would be amazed what a little change in the master bore makes at the wheels

 

It can be unbolted. Two bolts from the inside going through the bottom of the brake pedal mount and the other four in the engine compartment...

 

What car had a 5/8" bore master? My original was closer to a 2/3 being 21/32"...

 

Wow! Thank you so much for taking the time to do this. Once again I wish you were closer and I'd hire you to do this. Not sure any local guys know how to pull this off or have the patience to do it but I will show all of your suggestions as well as the other replies to the shop. They are the ones who suggested I take to the board to get info. As for the new assembly, not sure that bottom one would fit. There's very little room for a 14" long assembly unless it's cut in half or a third is cut off. The top one seems like it may def work. And if I understand correctly - the heavy wall tube basically protruding out of the wall creates a flat plane that the brake assembly can mount to which will bypass attaching it to an uneven wall?

 

Did the rest of you guys miss the fact that his pedal ratio is 2.4:1?

 

The two lower pedal assemblies are designed to have the excess cut off. They have multiple bolt holes to hang the pedal from, and you can even drill your own, where it fits best. Same goes for the dashboard brace.

The heavy wall tubing and long bolts get the booster attached directly to the pedal assembly, sandwiched with the firewall. They pass through the plenum wall, but that metal would then no longer be in-play.

Attaching the booster in this fashion applies the force delivered by the pedal, directly to the booster.

As it is now, the sheet metal of the plenum is what is taking that force.

I'd rather see 4 Grade-8 bolts taking that force, than some 18ga. sheet metal.

 

How did you move the clevis up? I know they needed to extend the pushrod on the booster, but not sure the clevis was adjustable on the factory pedal. Also, the pedal is about 2.4:1 ratio. I spoke with a couple guys that own 57 Buick Supers and have the same size dual boosters and said it stops the car better than the original power treadalvac so I didn't think it would be an issue.

 

Here is one of the Speedway units I installed in my 47 Lincoln. I went from a floor pedal to a hanging pedal using their kit. I cut off the channel where the pedal mounts so it fits under my dash, and bolted it to the dash. I tried several of the pedal mounting holes until I got one that felt right. I have 12" drums in front with 10" drums in back, and my pedal is easy, actually too easy. Not spongy, but easy to push. I may even remove the booster as I don't think I really need it.

 

Whoops, let me try that again.........

 

The last photo didn't go through. Can you try to upload again? I see you have residual valves on the lines. Mine doesn't have that, but the shop said they didn't see a reason to put them there because it would probably make the existing pedal even stiffer. Does that sound right or would those valves help?

 

Residual valves have no effect on pedal pressure. They are to keep seals preloaded (and to prevent drainback, if your master cylinder is under the floor).

If you have drums, you must have have them, of some sort, in the system.

 

I'm not sure I really needed them, they are used to keep a bit of pressure in the lines so the brakes work without a lag. They are usually in the ports of the master cylinder, but this being a aftermarket copy of the Corvette master cylinder, I wasn't sure if it would have them. They don't make the pedal any harder, the way they work is like a one way valve, fluid goes toward the shoes, but it won't allow all the fluid to return to the master, it retains a small bit of pressure in the lines. I think the pedal ratio is your problem, it makes it hard to press the pedal enough to move enough fluid to work the wheel cylinders. Think about a see saw..........if you get on the end of it, you can move the person on the other end easily. Slide up toward the pivot point, and it gets hard or impossible to move that other person.