combination valve with newschool GM Master?

[jetmech442]

I'll spare the background story unless it's needed of course, but I have a 2005 GM silverado Master cylinder going in place of my CPP(Classic Parts Performance) typical iron A-body master cylinder going in my 69 442. Disc front/back, so I think the only thing the the combination valve does is protect from loss of front or rear circuits.

Does anyone see an issue with this? do I even need the combination valve anymore? I'm not sure if the new MC's have this tech built into them... hoping someone takes me to school here or at least casually calls me an idiot before moving on.

I know that my pressure bleeder is useless now, it only flows if the pedal is pushed and held.

[jetmech442]

I'll answer and close this thread for anyone who finds it later. combination valves are still used with the new master cylinders. It serves to close the front or rear in case one of them springs a leak, the other will still function. I don't know if they all have proportioning valves installed or if the ABS takes over that feature, but for what I'm doing, I will keep the comb valve in place.



Cheers Shipmates.

[another69]

From what I can tell it seems like the combination valve offers another layer of safety? It has a piston that will block off the dead circuit. Can anyone confirm this?

[cdrod]

Jetmech:
Most combination valves for disc/drum cars after 1970 also have a residual valve to keep some pressure in the rear wheel cylinders. This keep the rear brake shoes closer to the drums so the front & rear brakes apply at the same time. If your running 4-wheel disc brakes you don't need the residual valve. I'm building a '72 442 convertible with C5 brakes on the front and 2001 Blazer discs on the rear. I installed a combination valve for a '70 Cutlass which doesn't have the built-in residual valve, PN PR-107 from Inline Tube. The '70 Cutlass had a separate residual valve installed in the rear of the car so the combo valve is just used to trigger the brake light switch and the emergency 2-circuit protection. Hope this was helpful.

Rodney

[another69]

Yes, the emergency 2 circuit protection is my concern. It looks to be built into the combination valve. Is this safer than running just the master alone? Or do newer masters also have this built in.

If you blow a brake line with a combination valve, it looks like the internal plunger moves, shutting off the blown circuit & triggering the brake light on the dash.

Is this redundant or neccessary?

[stab6902]

The differential pressure switch inside the combination valve is only used to trip the red "brake" malfunction indicator light on the dash. It does not switch off a leaking circuit. All tandem (aka dual bowl or dual pot) master cylinders have redundancy designed into them. If one circuit has a leak, you'll have longer pedal travel, but the other circuit will still function. It would be tough to miss the long pedal travel and need the "brake" light to realize there's a problem.

You will still need some kind of proportioning valve to ensure the rear wheels never lock before the fronts. More reading on the topic: https://www.freeasestudyguides.com/c...lve-brake.html