what size master cylinder bore
 

i just did a rear disc conversion on my 1972 k20 truck. the truck now has both front and rear disc brakes. the truck has power brakes. i think my master cylinder is shot. it just bubbles air into the reservoir when i apply the brakes. I see there is a 1 1/4 or 1 1/8 master bore available. I went down a rabbit hole trying to figure out which one to order. im more confused now than when i started. do i need to upgrade to the 1 1/4 bore now that i have dual disc brakes or should i stick with the 1 1/8 bore?

Re: what size master cylinder bore
 

Either should "work", I think 1 1/8th is pretty much standard with a booster and disc brakes.

Smaller bore = easier pedal, larger bore = more effort needed.

If you don't have abs to do the thinking for you when you have to panic stop then its better to have a brake system that requires more effort so you can more easily modulate the brakes to keep them from locking up. If the pedal is light/easy you'll just jab the pedal and lock them up and slide uncontrollably in a panic stop situation. A higher effort pedal and some practice stops to get a feel for how the truck reacts under very hard braking could be the difference between stopping in time or sliding into something you don't want to hit.

Re: what size master cylinder bore
 

I would as Tobin at KORE3.com. He is the brake guru.

Re: what size master cylinder bore
 

I would contact Tobin at KORE3.com. He is the brake guru.

Re: what size master cylinder bore
 

I agree with contacting Tobin. Are you using a booster? The proportioning valve needs to be correct for a disc/disc design as the rear discs require more volume of fluid than drums.

Re: what size master cylinder bore
 

Im running the stock booster for the truck. I think the booster works fine. The truck pulls good vacuum and the pedal is not hard. I havent changed the proportion valve yet. The kit manufacture says that it isnt necessary for their kit. I have been thinking of replacing the front and rear calipers. The front calipers have different sized bleeder screws so it makes me think that they are not the same part or brand? The rear calipers that came with the conversion kit have the integrated parking brakes... i found during research that they are hard to adjust and can cause pedal issues. So i want to swap them out with the jb6 calipers to see if that makes a difference. I also was planning on switching the porportion valve to a pv4 disc/disc version and upgrading the master to a corvette disc/disc type master. I think I can get all the parts from a cheap online part store for a little over 200 dollars. I just dont want to throw money at the problem and potentially not fix it. But on the same hand, Ive had the truck for almost 2 months now and havent been able to drive it due to brake issues. Im getting ancy lol


one other thing I forgot about. I unbolted the master from the booster to check for leakage and didnt see any. But when I tried to push in the master cylinder it was super hard and only went in a 1/4 inch or so. Is that normal? I thought you were able to bench bleed with a screw driver? or is the lines being hooked up causing alot of back pressure?

Re: what size master cylinder bore
 

Most calipers have different screw sizes on the front versus the rear. I have no idea why. I have done quite a few rear drum to disk conversions and I never used a proportioning valve. The rear never locked up before the front in any of them and they all brake fine. As for the rear parking brake, they are not hard to adjust. You have to use the parking brake to adjust them accordingly. Most people think you can just throw them on, pump up the brakes, bleed them, and you are set. This is generally not the case as they have a ratchet mechanism in them that pushes the pad out for adjustment. The ratchet is activated off the emergency brake. They usually adjust themselves as the pad wears, but it is always helpful to pull the e-brake occasionally. I never use a Corvette master. Nothing wrong with them, but I never needed them. The extra fluid isn't really needed except during the initial bleeding process. After the brakes are all bled, you won't be emptying that reservoir unless you break a line, then it won't matter anyhow.

You won't push the master rod into the master with the lines hooked up and the system bled out. If it were that easy, you wouldn't need a power brake booster.

Re: what size master cylinder bore
 

thanks for all your guys replies. Ill hold off on the master and the proportioning valve for now. Ill just try replacing the calipers for time being.

When I say the bleeder screw sizes didnt match up. the left and the right side calipers had different sized bleeder screws. I assumed one of the calipers was replaced at one time during the trucks life. I had issues bleeding one side of the front. I would barely crack the bleeder and the brake fluid would flow around the bleeder threads. i tried replacing the bleeder screw but it did the same thing. manual bleeding always produced bubbles. I put a goop of grease around the bleeders and tried to use a pneumatic bleeder to bleed the wheel. I think I may replace the caliper just to be sure its not leaking or introducing air when i bleed.

I think another problem is that I dont have the parking brake cable set up on the rear. Im trying to find an universal parking brake kit to operate the parking brake levers. So I might just throw on some cheap rockauto non parking brake calipers to see if it fixes the issue. If it does I can revisit the brake issue when I find a solution to the parking brake cables.

Re: what size master cylinder bore
 

The issue with proportioning valves is that the stock disc/drum unit has a residual valve. It is designed to keep some pressure on the rear lines, because the drums have return springs, which discs do not have/need.
Using the original will put that pressure into your calipers. It's only a couple of pounds, but it is there and can cause drag on your rotors.
Switching to a disc/disc is the way to go.
The original question about bore size is mostly about fluid volume for the same amount of stroke. Discs require more volume, but it may not be enough to require the change? Especially since the fronts do most of the stopping anyway.

Re: what size master cylinder bore
 

The residual valve is in the master cylinder on stock drum brake trucks.

A proportioning valve is just a proportioning valve- it limits rear line pressure keeps the rear brakes from locking up. I am not aware of any stock proportioning valves that has a residual valve built in.

Disc/drum systems have a combination valve that is commonly but mistakenly called a proportioning valve. They have a hold off valve in the front port, that “holds off” the discs from applying until line pressure rises enough to overcome the shoe return spring tension, that way the discs and drums begin applying at the same time to even out shoe and pad wear.. The rear port has a proportioning valve. I am not aware of any aftermarket combo valves that have residual valves in the rear port for the drums.

Re: what size master cylinder bore
 

I'm more confused now. lol. Maybe I'll start with the calipers and if that dont work do the master and proportion valve.

Re: what size master cylinder bore
 

not enough information,
whats the piston sizes of the calipers, front and rear.. 1 piston or dual piston calipers...

also, post the OE size of the pistons

Re: what size master cylinder bore
 

i think i got it working. i used a corvette disc/disc master. everything
seems good now