brake help !!!

[bagged70]

ok i am useing a mid 70 corvett mastercylinder and a mid 80 chevy truck PROPORTIONING valve i have good pressure at the mastercylinder but very little at the rear brakes it does not even move the wheel cylinders all my lines and parts are new but the proport valve is used is it bad or does the car master cylinder pressure to low to work the truck proport valve thanks for the help

[Longhorn Man]

off topic, but why would you use a car master cylinder on a truck that weighs a third more than the car that master cylinder was designed to stop?

[powell72]

Weren't Vette's disc/disc?

[bagged70]

It look nicer than the truck master cylinder no other reason but that. A nd yes thats why i ask the is a car cylinder less pressure, the frt brakes works fine

[Longhorn Man]

if it is disk/disk, then it'll be higher pressure and more volume than your wheel cylinders are designed to handle.

[bagged70]

its still drum in the rear when i bleed the wheel cylinders it drizzle out instead spraying out like it should guess ill just put a truck master cyl. on it and see if the problem goes away

[Longhorn Man]

I actually ment if the master cylinder was disk disk... my fault.
The actual problem at hand sounds like the proportioning valve needs recentered.

[bagged70]

ok, proportioning valve needs recentered? what do you mean i have no idea

[SCOTI]

Originally Posted by bagged70

ok, proportioning valve needs recentered? what do you mean i have no idea

There's a GM procedure where you compress a pin on the exterior of the prop valve to allow the internal valving to 'reset'.

To be honest, when my 74's prop valve was acting up I tried all the suggestions from our fellow board members to reset it. In the end, I wound up scavenging the new unit I bought from Inline Tubing for my 68's brake upgrade. Once I swapped the new one in, everything worked correctly.

Like Longhorn Man mentioned, I think you still might encounter issues if the master cyl is valved for a disc/disc set-up. Find a disc/drum unit & test it w/no other changes & see if there's any improvement.

[VetteVet]

Originally Posted by powell72

Weren't Vette's disc/disc?

Not until 1965

[Boog]

I believe there is a residual valve in the drum side of the master cylinders to maintain a slight pressure in the rear lines since there are springs to retract the shoes. The rear disc setups do not require this. Even changing the proportion valve will not make it work correctly.
You will have to use a m/c matched to the brake system on the truck to work correctly.

[chris71]

Originally Posted by Boog

I believe there is a residual valve in the drum side of the master cylinders to maintain a slight pressure in the rear lines since there are springs to retract the shoes. The rear disc setups do not require this. Even changing the proportion valve will not make it work correctly.
You will have to use a m/c matched to the brake system on the truck to work correctly.

i agree, you have to get a disc/drum MC if you have disc/drum, that vette MC is disc/disc. The MC and calipers/drums are designed to work together, on any vehicle, when you start swapping componants thats when stuff starts going wrong. the MC is designed (bore size) is designed to work with the volume capacities of the calipers, and the pressure as well, you would need more pressure theoretically to stop your truck than a lighter vette. if you have stock front discs i would reccomend getting a stock MC for disc/drum or getting an aftermarket kit that all the componants are desinged to work together for a vehichle of your wieght. brakes are the one thing you dont want to screw with. bad brakes equals wrecked truck

[Garagedude]

I was reading this and I am a little confused. I am doing a complete disk brake conversion, I have the wilwood disk brakes on the rear, and another kit on the front, I haven't finished the front yet. I know that I will have to change the proportioning valve. But will I have to change more because of all wheel disk brakes.

[chris71]

Originally Posted by Garagedude

I was reading this and I am a little confused. I am doing a complete disk brake conversion, I have the wilwood disk brakes on the rear, and another kit on the front, I haven't finished the front yet. I know that I will have to change the proportioning valve. But will I have to change more because of all wheel disk brakes.

depends what you are swaping from and what you have now, you will need a disc/disc MC. brakes are not real easy, you just cant take parts off different cars and hope they work together, some get lucky, alot dont.

[bagged70]

i went and got that ugly truck master and would you know it it works just fine now thanks for the help

[Garagedude]

I am using a 12 bolt rearend out of a firebird, I put the wilwood disk brakes on it. For the front I am using a kit off performance online it uses chevelle rotors and camaro calipers.

[Sport/Truck]

Originally Posted by bagged70

i went and got that ugly truck master and would you know it it works just fine now thanks for the help

Here's one thats pretty and eliminates the portioning valve.
http://www.abspowerbrake.com/6772chevytruck.html
s/t

[chris71]

Originally Posted by bagged70

i went and got that ugly truck master and would you know it it works just fine now thanks for the help

glad to hear you got it working, you can always upgrade to a better setup in the future, there are a bunch of good companies out there like ssbc, wilwood, baer, that do performance upgrades and packages, feel free to give these people a call if you are interested, i would, they have a wealth of knowledge and want to give you info so that you can buy there stuff.

another thing i remembered is that vette 4 piston calipers are fixed (non floating) which means they need like a 100psi less brake pressure to operate than single piston floating calipers (like on our trucks), the MC would be designed taking this into consideration, and i have heard that even though a 4 piston caliper seems bigger as far as volume is concerned, it takes less volume to move 4 small pistons than one huge one, once again the mc would be designed around these volume requirements

[chris71]

Originally Posted by Garagedude

I am using a 12 bolt rearend out of a firebird, I put the wilwood disk brakes on it. For the front I am using a kit off performance online it uses chevelle rotors and camaro calipers.

does the performance online kit come with a MC and prop valve, if so these two would need to be for a disc/disc setup. i am assuming that both front and rear calipers are single piston. if this is the case and the kit was designed for 67-72 trucks like i think it may (from veiwing performance online's website) then you should be ok. if you are still confused or doubting, i would call performance online and see what they have to say, if they sell this stuff they "should" know something about different setups.