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- - Brake Booster Version? (https://67-72chevytrucks.com/vboard/showthread.php?t=859712)

Brake Booster Version?

Now that I have installed a dual chamber master cylinder I would like to upgrade to power brakes. From what I have read for 64-66 the master cylinder was the same regardless of manual or power so I should just be able to add a booster.

However when I look at the booster for those years there are 2 versions (Horizontal Bracket & Vertical Bracket).

Is there a reason for 2 different versions? Can I install either of them?

Re: Brake Booster Version?

The only differences I see:
1. 37 comes with a bracket while 35 just comes with the studs to install another bracket - stock or one like Captain Fabs here on the site.
2. 35 comes with a brake pedal rod extension and 37 does not.
3. 35 is "cast iron" and 37 is listed as "steel".
4. Finally, 35 is listed as "remanufactured" and 37 is listed as "new".

That all said, you can dial in your setup a bit with booster selection. I went with a dual diaphragm booster that was slightly larger from 2000 something S10 based on my master cylinder bore, pedal swing, and drum/disc setup. I used the 60-63 bracket Captainfab sells to get me a hole for a clutch master cylinder as I am switching to a hydraulic setup versus the mechanical linkages that are stock.

Re: Brake Booster Version?

What diameter master cylinder did you buy?

Manual brakes generally use a 1" bore.

Power brakes generally use a 1.125" bore.

The pushrod pivot point on the pedal arm also needs to be in the correct location for power vs manual. The stock 64-66 pedal ratio for manual brakes is 6.25:1, power brake ratio needs to be around 4:1 so you'll need to move the pivot point by drilling a new hole farther down on the pedal arm.

If you already have a 1" master cylinder and the pivot point is still in the original manual brake location a booster isn't needed, it'll stop great without it. That's how my '66 is set up, stops great even when towing.

Re: Brake Booster Version?

I believe the 4 wheel drum masters were all 1.0" bore for manual and power even into the second gen dual masters.

Quote:

Originally Posted by theastronaut (Post 9394030)

Re: Brake Booster Version?

Quote:

Originally Posted by 1964FarmFreshC10 (Post 9394045)

I believe the 4 wheel drum masters were all 1.0" bore for manual and power even into the second gen dual masters.

The problem with just adding a booster and not modifying the pedal ratio or increasing the master cylinder size is that it makes the pedal too easy and stopping with any kind of ability to modulate the brakes becomes difficult.

I posted the math on booster assist, pedal ratio, and master cylinder bore size in another thread, I'll copy/paste it here. It's fine to make the brakes more powerful, but you also need to do it in a balanced way so they're still able to be applied in a controlled manner. Without abs you don't want them to lock up easily.

Quote:

Manual pedal ratio is generally around 6:1, power brakes are generally around 4:1. My stock '66 C10 has a 6.25 ratio. The higher the number, the more leverage the pedal has on the master cylinder.

When you convert to power brakes you do two things to take away braking power- reduce the pedal ratio by about 35%, and you generally go from a 1" bore MC to a 1.125" bore MC, reducing output PSI by 20%. The booster is supposed to more than make up for that, right?

With a stock 6.25 pedal ratio and 100 pounds of input force on the pedal you get 625 pounds of force into the MC, which is hydraulically multiplied via the MC's piston surface area of .785 square inch. 625 pounds in turns into 790 PSI at the caliper if you have the typical 1" bore MC.

With a power brake conversion, 100 pounds of force on the pedal gets multiplied by 4 via the pedal ratio for 400 pounds into the pushrod/booster. CPP says a 7" dual diaphragm booster adds 493 pounds of force for 893 pounds of force into the MC. The typical 1.125" MC will turn 893 pounds of input force into the same 893- the bore size has 1 square inch of area so it doesn't multiply the input force any so you have 893 psi at the caliper. This is about 13% more psi than the typical manual setup ** BUT ** this is assuming 23" of manifold vacuum. How many trucks actually make 23" of vacuum? That 13 % advantage quickly fades in the real world.

What happens if we stay with manual brakes but drop down to a smaller master cylinder bore to multiply input force more? With a 15/16" bore you get 905 psi at the caliper, and 7/8" would have 1041 psi. Brake pedal input to output is totally linear and not variable with manifold vacuum, and they work 100% the same with the engine on or off. It's cheaper and easier because pedal modifications aren't needed.

But, everyone says you need power brakes "because they stop better" and brake companies need you to keep believing that so you keep buying their aftermarket "upgrade" parts...

The one benefit to power brakes is that they can have less pedal travel than manual brakes, because of the shorter pedal throw due to the lower ratio, and because the bigger MC bore displaces more fluid for the same amount of travel. Is this an issue in the real world? Its not on my '66 with the stock 6.25 pedal ratio, 1" bore MC, the usual squarebody D52 12" discs in front, and stock rear drums. The pedal drops a normal amount and pedal effort is pretty normal too. If you have air in the lines then you'll have more pedal travel since air is compressible, but you shouldn't have air in the lines if you bleed them correctly. If you have bigger/multi-piston calipers then you'd need to design the whole system around the amount of volume/pressure they need. But for the typical D52 disc setup, a manual 1" bore MC works great and makes plenty of hydraulic pressure without the pedal travel being any more than normal.

Re: Brake Booster Version?

This vid shows how well correctly set up manual brakes work compared to a mis-matched power setup. Power brakes can work, but you need to know what you're doing to make sure they work correctly.

https://www.youtube.com/watch?v=EJIbq3PK_tk

Re: Brake Booster Version?

Thanks theastronaut both posts very interesting and gives food for thought.

Quote:

Originally Posted by theastronaut (Post 9394052)

Re: Brake Booster Version?

Quote:

Originally Posted by theastronaut (Post 9394051)

This is pure gold! Thanks for the maths!

Quick question, using your power brake example that evidenced 893 lbs of pressure, is this then split at the proportioning valve at a 40/60 ratio back to front?

So, 893 lbs of total pressure results in the front receiving 535lbs (60%) and the rear receiving 357.2lbs (40%) at 23 inches of manifold vacuum?

Re: Brake Booster Version?

Quote:

Originally Posted by 1964FarmFreshC10 (Post 9394058)

Thanks theastronaut both posts very interesting and gives food for thought.

No problem! There is a lot of wrong/bad info out there on brake systems, and brakes are too important to get them wrong! I post what has worked for me with the math to back it up; magazines and brake companies are in the business of selling parts so of course they push "upgraded" parts.

Re: Brake Booster Version?

Quote:

Originally Posted by scottofksu (Post 9394200)

Both front and rear circuits have the same pressure since both circuits are operating from the same size master cylinder bore, so the output pressure is the same front and rear. The proportioning valve limits the maximum output pressure to the rear to keep them from locking up. They have the same pressure during normal driving, but if you slam on brakes the proportioning valve limits total rear pressure. On factory drum/drum or disc/disc setups without a proportioning valve the staggered wheel cylinder/caliper bore sizes are made to balance front to rear braking power to keep the rears from locking up first. On modified systems (or even modified/staggered wheel/tire sizes) its a good idea to use an adjustable rear proportioning valve so you can get the most out of the rear brakes before they lock up. The generic "one size fits all" combination valves that come with most brake kits aren't ideal except for the one application they were initially engineered for, and may not let the rear brakes stop as hard as they could, or they might allow too much rear pressure and still lock up the rear too early.

There are systems that vary front and rear pressure- race cars have brake pedal boxes that have two master cylinders, one for front and one for rear, and an adjustable balance bar that applies more or less pressure from the pedal into each master cylinder. That setup would vary pressure front to rear, and make for easy adjustments to dial in the front to rear balance. Some even have a dial on the dash that will adjust the balance bar on the fly.

https://tiltonracing.com/product/900...edal-assembly/