1970 C20 Disc Brake Swap - Vacuum Booster Question

[ScottN795]

I just finished swapping the front brakes on my 70 C20, however I'm having an issue with an extremely hard pedal and not much in the way of brakes. A quick overview of what I did: swapped the spindles for stock 72 Disc brake spindles, replaced the calipers with new stock 72 calipers (rebuilt from O'Reilly's) new upper and lower ball joints, new inner and outer tie rod ends with new tie rod adjustment sleeves, resurfaced rotors and new pads, new rubber brake hoses. I also replaced the M/C with a new 72 (rebuilt unit from O'Reilly's) for disc brakes. I also replaced the proportioning valve with a CPP not adjustable disc/drum unit.

The only thing I did not replace was the original stock 8.75" vacuum booster that originally was for the stock power drum brakes. I did shorten the rod from the booster to the M/C to the length it "appeared" it needed to be based on the throw of the new M/C, however it was really my best guess. I think my issue is the overall distance the rod is travelling into the new M/C is not correct, but I'm not sure how to figure out what it needs to be? I was also thinking of just biting the bullet and purchasing a rebuilt brake booster for a 72 C20 and just be done, (the 72" booster lists as a 10.81" unit) but that's another $100 or so and it's not a stocking item locally.... I have always believed that a brake booster is pretty much a brake booster (choices of single and dual diaphragm and bolt pattern aside) and I expected this to work...

Thoughts?

Thanks,
Scott

[ScottN795]

I forgot to mention that this is a running, daily driven truck and the power drum brake set up worked well, so I don't feel the brake booster is "bad" or that engine vacuum is an issue. It is a stock 350 and runs well, without any other issues.

Thanks,
Scott

[geezer#99]

I might be wrong but I think the rod between the booster and master needs to have about a 1/16 inch of free play.
Have you checked the master is the same depth where the rod contacts it? There are different ones.

[ScottN795]

Thanks for the input. In measuring the original drum brake m/c and the new disc brake m/c, it appears the disc unit is about 7/16" shallower than the original drum m/c. That is the approximate amount that I shortened the rod between the brake booster and the m/c. While bleeding the brakes, fluid did pass to all 4 corners, however I'm not sure if I'm getting full engagement, if that makes sense? How would I check to verify that?

[geezer#99]

Jack up each wheel and while spinning the tire work the brakes.
That might let you see how well they engage.

[geezer#99]

Here’s a thought from way out in left field!
I’ve encountered hard pedal like you describe and checking and changing the pedal ratio helped a bunch. I drilled a new hole one inch lower in the pedal and attached the push rod there. Power usually needs a 4.5 to 1 pedal ratio.
That gives you a longer stroke on the master.

[ScottN795]

That is a great idea! That would provide the additional throw likely necessary to create the travel needed in the m/c. I'll try this later today when I have a chance to work on the truck.

Thanks!

[ScottN795]

I was able to adjust the brake pedal rod under the dash and about 3/4" of throw, with no change whatsoever....

[geezer#99]

Could still be just the booster push rod.
Found this.

http://67-72chevytrucks.com/vboard/s...d.php?t=498293

How much vacuum does your motor produce?
Is the booster check valve and hose old or new?

[ScottN795]

That's great info and exactly what I was looking for to understand how to check the rod length. Not sure if it's good news or not, but mine seems to be good. I also checked and the engine is making about 17" of vacuum at idle, and when I step in the brakes there isn't any change.... I'm going to re bleed the system to make sure I didn't idn't miss air in it somewhere. Thanks for hanging in there with me on this.

[geezer#99]

It can be frustrating.
Here’s another link on holding in the button on the front of your prop valve. Helps the bleeding process.

https://67-72chevytrucks.com/vboard/...=Proportioning

[Uglykato]

Just subscribing for when I swap mine over to disc.

[MACO]

After I did the swap on my 69 C20 I had a hard pedal also. When I switched to a OEM disc brake dual diaphragm booster with bracket and rod the pedal was better. Lastly I ditched the CPP combination valve for a OEM one and the pedal and brake power is great. I am not a fan of the brass combination valves, 1st one leaked right away and the brakes never felt great.

[ScottN795]

Thanks for the help and support guys as I worked through this. I ended up replacing the vacuum booster with a stock 72 C20 dual diaphragm unit (new - rebuilt unit from Napa - $112) changed the firewall mounting brackets to the longer style (luckily I had a set from a 72 C10 in our shop). I believe I still have a small issue with the rear driver's side wheel cylinder, however the truck stops like it never has before, and with all of the new steering pieces in the front end, it drives much better as well.

As I have a chance, I'll list the parts (with part numbers from O'Reilly's and Napa) that I used for my swap in case it will help others out in the future. I did run into ball joint and tie rod issues with parts that "should have" worked, but didn't. So maybe the list can help others save a few trips to the parts store.

I should have the list up in a day or so.

Thanks again,
Scott

1970 C20
1965 Suburban
1972 Nova SS
(other stuff...)