Master cylinder recommendations for C5 front and factory drum rear.

[CreepinDeth]

I've been running a C5 front , and factory drum rear in my 2nd gen for a few years now.
I installed a 9" dual diaphragm booster, and a 1979 Vette master which is for Disc/Disc
with an Adj prop valve on the rear 1/4" line. Front lines are 3/16".



I thought I was told it would be okay, but these days I'm not so sure.
I keep running into brake issues after I get it bled and working again.


It ran just fine for 1 whole season with maybe 1000 miles put on it.
This last season I had a leak in my front circuit which bled my MC out dry.

I couldn't find a leak ANYWHERE.....obviously I didn't look hard enough but I expected
a puddle and there wasn't one. So I filled it and rebled it. Late last season it bled
dry again. So I'm at a loss and I'm wondering if this MC is the problem.

Any recommendations ??

[Apogee]

Fix your leak first...please. These are your brakes and the only reason you should "lose" fluid in the reservoir is due to brake pad wear, where you're not actually losing fluid, it's just moving from the reservoir to fill the void behind the caliper pistons as they extend to make up for pad wear. The fluid could be leaking onto your headers or something so you're just not seeing the leak. When you have bleed the brakes and everything feels good, you should be able to apply the brakes firmly and the pedal should not drop over time. If it does, you have either an external leak (which is sounds like you have) and/or an internal leak allowing fluid to bypass the MC seals and return to the reservoir.

As for your master cylinder, the 1979 C3 Corvette unit is a 1-1/8" bore MC as my understanding is that they only offered power disc/disc brakes on the 1978-1982 C3 Corvettes. Prior to that, there was a 1" bore disc/disc MC used on the manual brake applications. My first thought is that you should be complaining about a very high, firm pedal and excessive pedal effort to stop the car...but you're not, which is interesting. Considering the C5/C6 Corvettes use a dual-9" booster with a 1" bore master cylinder, that's about a 25% difference.

Hard line size makes practically no difference in terms of performance and has no impact here.

My recommendations would be as follows:
1. Stop driving it.
2. Find your leak.
3. Replace your master cylinder with a 1" bore unit. Unit can be disc/drum with 10 psi residual pressure valve built into the rear outlet port or disc/disc with an inline 10 psi residual pressure valve for the rear wheel cylinders.

Tobin
KORE3

[CreepinDeth]

Fix your leak first...please. These are your brakes and the only reason you should "lose" fluid in the reservoir is due to brake pad wear, where you're not actually losing fluid, it's just moving from the reservoir to fill the void behind the caliper pistons as they extend to make up for pad wear. The fluid could be leaking onto your headers or something so you're just not seeing the leak. When you have bleed the brakes and everything feels good, you should be able to apply the brakes firmly and the pedal should not drop over time. If it does, you have either an external leak (which is sounds like you have) and/or an internal leak allowing fluid to bypass the MC seals and return to the reservoir.

I understand all that with the lower pads = less fluid many people overlook.
The problem is my front reservoir went bone dry 2x now. So it absolutely has to be an external leak.
I don't drive the car much, in fact 2011 only saw 2 or 3 cruises, but the final cruise was after I rebled and filled it.

Then it leaked out on me again after maybe 100 miles and I lost pedal and parked it permanently until I had time to figure it out.
I've been thinking maybe my 78 brake lines need to be replaced.....but wasn't sure if the MC was the issue.

BTW the pedal worked fine for me when it was bled and pressure restored. Wasn't too high or hard with excessive pressure required.

My recommendations would be as follows:
1. Stop driving it.
2. Find your leak.
3. Replace your master cylinder with a 1" bore unit. Unit can be disc/drum with 10 psi residual pressure valve built into the rear outlet port or disc/disc with an inline 10 psi residual pressure valve for the rear wheel cylinders.

Tobin
KORE3

Tobin, what factory MC would fit this description , do you know of any?? I'm trying to stay with factory stuff here since it's less $$.

Thanks again.

[Apogee]

You know what they say, location, location, location...if you live where rust is an issue, than 35 year old brake lines are probably becoming an issue if not a downright hazard. That's as good as any place to start looking for leaks.

As for master cylinders, there are 1" and 1-1/8" bore disc/drum versions of the second-gen F-body master cylinders...just need to figure out which one you want and run it. I'm assuming your current master cylinder has the shallow pushrod hole and that your booster has the corresponding short pushrod, so that needs to match as well.

Tobin
KORE3

[CreepinDeth]

Yeah.....I think I'm going to have to pull the lines and replace em. Thanks again bud.

[scott1956]

Look inside the car at the rear seal of the MC where the brake pedal pushrod goes in from the brake pedal. I have seen them leak there and dump fluid inside the car under the carpet.
Scott