Low pedal after disc conversion...? HELP!

[DonMan]

I am helping a friend with a front disc conversion on his `67 Nova.
We modified his drum spindles and used the rotors, calipers, hardware, etc...from a `70 Chevelle. He has manual brakes, so we used the master cylinder from a `75 Camaro with manual discs. All parts are brand new. The car stops fine and will lock the front tires up upon hard braking. What concerns us is that the brake pedal travels almost to the floor. It rests in the normal postion, but gets quite low when braking. These Novas did not use a proportioning valve and we are not using one either. I think this is our problem. If so, do we just need an adjustable valve in the system or must we use a factory type?
Any / all advice is welcome.
Thank You!

[Rick Dorion]

The pedal should be firm. What's the bore size on that MC? I assume you now have disc front/drum rear? On my car with C4 fronts, I use a 78 malibu MC with a 15/16" bore in manual mode. Pedal is high and firm. Let us know more. You could also try blocking off the MC port one at a time and when the pedal is firm, you'll have narrowed down the circuit.

[Q ship]

used the rotors, calipers, hardware, etc...from a `70 Chevelle. He has manual brakes, so we used the master cylinder from a `75 Camaro with manual discs. All parts are brand new. The car stops fine and will lock the front tires up upon hard braking

The master cylinder sizing is fine. Try rebleeding the brakes. Sounds like there is still some air in the system. Let us know what helps!

[DonMan]

I`m going to suggest we bleed the brakes more.
I`ll let you know the results.

[Zefhix]

You could always try the power brake hole on the brake pedal linkage and see if that picks up some of the slack even though you're running manual discs.....

[fuzzyonion]

if you are using 70 chevelle calipers on a '67 nova, you need a 1 1/8" master for a good pedal.

[HILROD]

You need to install a prop valve the brakes will act crazy until you get rid of the stock junction.

[fuzzyonion]

actually, the nova never had a prop valve, except maybe on some cars, and the calipers had a smaller piston area than the single piston jobbies. this made the front to rear ratio even closer than it would be with the chevelle calipers. a prop valve is nice, but it will probably be adjusted all the way out to where it doesnt do much.

the reason for the larger master is that the 2nd gen camaro had a lower pedal ratio than the nova, and the nova originally had smaller calipers (they only looked bigger). the total ratio between the cylinders and the pedal remains close to the same. if it changes much, you get a low pedal, or a hard pedal, depending on which way you go.

total front ratio on all manual disc brakes is nominally between ~42-45:1 (caliper area/master area)x(pedal ratio). any different, and the pedal may be either too low, or too hard. this holds true for all models.

the manual pedal ratio of the 67 nova was the same as the 69 camaro, which used a 1 1/8" master with those calipers. prop valve was used sporadically until 1970 according to my sources. AMA specs do not list one until then. later models all had them, and some added larger rear cylinders.

the stock junction for the 67 was the same part# for disc and drum 67-69 A,G,X,F according to my research, and contains no proportioning or metering valve.

http://www.outlawdiscbrakes.com/faq.html
http://www.grc.nasa.gov/WWW/K-12/Win...principle.html