[Craig Berkheimer]

I replaced the stock master cyl. on my 68 Firebird with a new factory replacement designed for a 74 Nova.(disc front, drum rear, manual brakes) The reason I did this is for the bigger 1" bore. I understand that this should be a bolt on deal.
I have factory drums up front and wilwood disc in the rear. The master cyl. was bench bled and installed. The brakes were bled using a vacume bleeder. Everything seemed to go great. I then tested the brakes and had none! Whats up? Should I back up and put the original master cyl. on? Is the stroke different for the Nova master cyl? (meaning do I have to change the pedal rod)
If anyone has made a similar change please give me some help.Thanks

[Craig Berkheimer]

I replaced the stock master cyl. on my 68 Firebird with a new factory replacement designed for a 74 Nova.(disc front, drum rear, manual brakes) The reason I did this is for the bigger 1" bore. I understand that this should be a bolt on deal.
I have factory drums up front and wilwood disc in the rear. The master cyl. was bench bled and installed. The brakes were bled using a vacume bleeder. Everything seemed to go great. I then tested the brakes and had none! Whats up? Should I back up and put the original master cyl. on? Is the stroke different for the Nova master cyl? (meaning do I have to change the pedal rod)
If anyone has made a similar change please give me some help.Thanks

[KEN CROCIE]

Try installing a residual pressure valve @ the front brake line outlet on the MC. The 68 had one built into the MC but the 74 disc/drum does not.Try a brake shop if the local parts stoe doesn't have it

[Craig Berkheimer]

After I pressure bled the system I have brakes now. Should I sill go with the residual pressure valve?
I put the wilwood disc on the back first because I got a really good deal on them. I still have the front stock Drums. Is this a problem? Will I at least have a better braking system than before?
Thanks for your help Ken.

[KEN CROCIE]

As you have already found out , the brakes will work fine without the RPV.If you feel there is a little too much pedal travel before the "slack"is taken up , the RPV will help.I suggest you install an adj. proportioning valve in the rear brake line to optimise the system you have now ,and then the future 4 wheel disc system.
Ken

[GP-K]

Craig,

The combination of drums on the front & disks on the rear is a weak one at best. The Wilwood disks on the rear will be money wasted until you upgrade the front brakes to disks too. In addition, the '74 style master cylinder may not be the best choice for a system that will be eventually disk/disk brakes, depending on which front disk brakes you select.

The drums need far less fluid displacement than the disks to effect proper brake operation. The larger master will tend to make the fronts grab harder and quicker, and you may end up with no braking action from the rears at all, even with a proportioning valve. This may not be completely unsafe, but certainly it is far, far from ideal. No car ever left the factory with drums on the front and disks on the rear. I would not recommend driving the car until you have a matched component braking system.

Proportioning valves, since they are intended to be used in line on the rear brakes, are used to REDUCE the amount of braking to the rears.
To add insult to injury, the factory front disk/rear drum master cylinder also has a valving bias to the front, so the combination of front drums with aftermarket rear disks would be a pretty unbalanced and ineffective system overall. I would NOT recommend using a proportioning valve in the front brake lines under any circumstances.

The '74 disk/drum master cylinder will most likely not move adequate volume at proper pressure to the rear disks, when combined with drums on the front. That master cylinder was intended to be used with the huge Moraine single piston front calipers used on most GM cars. You may be ok with that master if you use GM single piston fronts.

When GM used rear disks on the pre-'79 cars (ie. corvettes, '67 & '68 F bodies), they used 4 piston calipers (like the wilwoods) on all 4 wheels, which had smaller individual piston areas, and gave adequate braking action with their matched master cylinders. The later '79-'81 4 wheel disks used the moraine single piston calipers on all 4 wheels, with a master cylinder which had significantly different internal valving, and fluid capacity.

Hope this helps.
Good Luck,

K.