Disc conversion nightmare

[speED]

Hello,

I'm new to the forum, in fact this is my first post. Currently my brake system seems to be my worst nightmare. Recently converted my rear drums to disc with advice from another forum and a friend who use to run an oval car. Starting to feel as though I should have bought a kit and spent 3x the money. The car ran for years with disc in the front and drum in the rear with no complaints. Problems started once I changed rear to disc. The following are the components I'm using:

69 Corvette manual master cylinder
original disc/ drum proportioning valve
Front: small GM style caliper (79 Monte Carlo style)
3/16" brake line
Rear: same caliper as front with AFCO caliper brackets
2 psi residual pressure valve between master and proportioning valve
1/4" brake line form master to rear split
3/16" line across rear to rear caliper hoses

Problem: pedal has no feedback/ feel
When I step on the pedal it does not become firm untill pedal is depressed 2". Both front and rear brakes are applied firmly after 3/4" of pedal travel but you would never know based on brake pedal feel. I believe the car will stop, but think it will have a very weird feel. I would expect the brakes to feel super sensitive.

Thanks in advance,
ED

[gsxrken]

I can't tell from your post whether the 'vette M/C was used previously when you had rear drums. If it wasn't, make sure it has the same pushrod depth as your orignal m/c, as this can drastically change pedal feel. Also, make sure the new M/C had a bore no larger than 1"; manual brakes need one inch or less bore to stop a 4 wheel disc car well.
But it does sound like you still have air in the system. Technically you don't need the residual valve, and the prop valve should be adjustable to dial in the balance, but neither should affect your pedal firmness. (Most "systems" stock or aftermarket have about 50% more piston area in the front to act as a built in proportioning valve, so you will likely need an adjustable prop valve.)
Perhaps you need to bench bleed the M/C and then chase the corners again looking for air.

[speED]

Actually I did not specify that the car is a 67 Camaro.
No this is not the master cylinder used with the drum brakes, I chose the Corvette master since it came with four wheel disc as a stock option (believe it is an 1-1/8" dia.). I feel extremely confident there is no air. I clamped each brake hose at each caliper and the pedal was hard. I did not check pushrod depth and I will also check master piston dia. I was going to change to an adjustable proportioning valve after I test drove car also thinking that it does not affect pedal firmness.

Thanks
ED

[MrQuick]

Hello,

69 Corvette manual master cylinder what size bore?
!!ORIGINAL DISC/DRUM PROPORTIONING VALVE!! try a 3rd gen disc disc prop/combo valve or a full adjustible type
Front: small GM style caliper (79 Monte Carlo style)
3/16" brake line +1
Rear: same caliper as front with AFCO caliper brackets that large piston is going to require a disc/disc prop/combo valve
2 psi residual pressure valve between master and proportioning valve
1/4" brake line form master to rear split +1
3/16" line across rear to rear caliper hoses +1

Problem: pedal has no feedback/ feel
When I step on the pedal it does not become firm untill pedal is depressed 2". Both front and rear brakes are applied firmly after 3/4" of pedal travel but you would never know based on brake pedal feel. could be combo prop/combo valve or push rod length problem.

I believe the car will stop, but think it will have a very weird feel. I would expect the brakes to feel super sensitive. should not feel super sensitive, firm with a delayed touch .

[speED]

I checked the master cylinder, piston size is 1" dia.
Did not get chance to check pushrod depth, but will check.

MrQuick,
"try a 3rd gen disc disc prop/combo valve or a full adjustible type "
"that large piston is going to require a disc/disc prop/combo valve "
Will a fully adjustable proportioning valve from Willwood be adequate?

[MuscleRodz]

First thing you need to chenge is the prop valve. It is wrong for a disc/disc combo. I would also reduce the bore size on your master to 1" as mentioned above.

Mike

[speED]

Confirmed, master cylinder is 1" dia.
Will a proportioning valve from Willwood be sufficient?

[MrQuick]

Agreed drop the bore down. I haven't had any luck with the adjustable ones but give it a shot.

[jknight16]

Not sure if this is something you'd be interested in but i recently found this SSBC distribution block with a built in proportioning valve, might be a simple way to solve a couple variables in your problem

http://www.stainlesssteelbrakes.com/products/detail/5505/?make=Street%20Rod%20Accessories&model=Prop+Block%3A+Adjustable+Proportioning+Valve +and+Distribution+Block+All-In-One&year=

[Skip Fix]

All Vettes after 64 had 4WD. The HD sytems used a 1 1/8 MC the "plain" used 1". Reember also especially with alot more piston size you need the volume of a larger master also to move the piston. Drum wheel cylinders need ALOT less volume to move them. The Vette MC I have to use forone car deep recess for the pushrod, darn near 1".

You didn't mention if it actually stopped well eventhough it feels bad. Too much pedal ratio can still let you stop but loose the feel.

My Trans Am rear calipers have a similar sized rear piston has just a T in the front line and adjustable prop valve in the rear and stops great. Origially I had too much ratio pedal and it stopped good but had no feel.

[6'9"Witha69]

The 1" bore MC can work, the Combo valve (originally on the Disc/Drum cars) is the weak link here. Replace with a standard disc/disc distribution block and prop valve in the rear or use a T for the fronts and a prop valve in the rear.

[speED]

Alot of information! I'm going to try your suggestions and see if I have any luck. Hopefully I can spend time on the car this weekend.

Thanks!

[1FstChevy]

Yup a Wilwood or SSBC prop valve (along with others) will work fine.

I have the polished finsih SSBC unit with the pressure gauge.

[speED]

Started re-plumbing front lines today. I will be doing rear next weekend with Willwood proportioning valve. I let you know how it turns out. Borrowed a hydraulic flaring tool from a friend, if you have never used one you are missing out.

ED

[MonzaRacer]

Actually disc need less volume and more pressure rise.
Reason being the drum brakes have to move a lot more over disc. Disc only have to move a few thousands of an inch to activate.
I also dont like using the stock 1/4 in rear lines if it has that size. The reason is that drum brakes need more volume over disc.
You may even need to to go down to a 7/8 master cyl.
Good Luck.
Lee

[MrQuick]

I agree, 7/8" might go better. Also 1/4" to tee to 3/16" lines out.

[70TWO NOVA]

I just would have bought a kit, heres a good place if you are interested: https://dfbrakes.com/default.asp

[speED]

I just would have bought a kit, heres a good place if you are interested: https://dfbrakes.com/default.asp


If you only knew how many times I said the same thing. It has now become a challenge, I will probably not get a good night sleep untill the car is running again.
Thanks for the link.

[speED]

Plumbed new 3/16" lines for the rear and added a Willwood adjustable proportioning valve. Pedal height is higher and the feel is also tighter. I will be test driving in the next week after I finish a little suspension work.

Thanks for help.

[Chad-1stGen]

The master cylinder you are using comes in a number of kits and in fact is the one I'm using on my car. Factory vettes in that year range used 1" for the manual m/c and 1 1/8 for power. Personally I'm using the MANUAL m/c with a power booster (booster had correct rod adapter). Witht he car off brake effort is excessive though I'm using the power brake hole on the pedal.

Speaking of which make sure you are using the right hole in your pedal. I believe it's upper for manual, lower for power but maybe someone else can confirm since I'm going on memory atm.

As already mentioned you need to ditch that proportioning valve. There are relatively cheap disc/disc 5 port valve that retain the brake waring light otherwise you can use an inline adjustable on the rears and plumb the fronts direct off of a T or Y block.

I didn't change the front to rear line and currently the 1/4" line is working.

Oh one other thing. I don't think you want to use the residual pressure valve. They don't typically come in kits that use that M/C so unless for some reason the calipers you are using require it (not familiar with them) I'd ditch it.

[6'9"Witha69]

Speaking of which make sure you are using the right hole in your pedal. I believe it's upper for manual, lower for power but maybe someone else can confirm since I'm going on memory atm.You are correct.

[speED]

Finally drove car! Brake pedal is still a little low, but not that bad. Cannot lock-up brakes. 1" bore is apparently to big. Before I change master to a smaller bore I wanted to get feedback. Should I look into the step bore master mentioned in another post? Sounds as though it will possible solve my effort and slightly low pedal feeling.

Ed

[MonzaRacer]

Just source a 7/8 disc disc master cyl and give it a try it should raise the pedal feel drasticly. as the force you are applying is going to build peak preasure faster and giving you a better feel.
Alos do you have residula pressure checks all the way around (ie front and rear if not the fluid has to rebuild "take up" pressure before you get any movementy and the fact that they kee pthe pads a tad closer to the rotors.
I am goingto work out a set of rear disc for my Monza soon but gotta get it running first.

[speED]

Seems some people are for the residual pressure valve and others feel it is not needed. If I change to a 7/8 disc/ disc master does anyone know year, make, and model of donor vehicle so I can tell my auto parts guy?

Thanks,
ED

[strangler]

I would also like to know of the application.... Anyone?

[shep]

Raybestos # MC 39037 (R.H.) (cast iron) 7/8 bore
'78-81 Ford Fairmont, Granada, Futura, Zepher (Mercury)

Raybestos # MC 39531 (R.H.) (aluminum) 21mm bore
'81-86 Ford T Bird, '82-86 Mustang, '83-86 Marquis (Mercury)

Raybestos # MC 39310 (R.H.) (cast iron) 7/8 bore
'81-83 Ford Escort, Explorer, Lynx, Mark 7 (Lincoln)

Raybestos # MC 39027 (L.H.) (cast iron) 7/8 bore (L.H.)
'76-80 GM Monza, Sky Hawk, Starfire, Sunbird

[speED]

Shep,

Thank you,
ED

[MonzaRacer]

Uh pretty sure that none of those are actually speced for disc/disc
I guarantee the Monza isnt.
And I would take a look at a later disc/disc 98 S10Blazer as it has a 2 piece spindle I have thought about running on my Monza and it is a drop in for Gbodies and is upgradeable to Vette stuff so I would bet it would work well with disc/disc.

[speED]

Concluded I have the low drag calipers and therefor will need a step bore master

ED