Brake hold-off valve?

[TonyHuntimerRaceHome]

Hey guys,

In my research I'm finding some serious inconsistencies.

It appears that the valve under the master is listed as a hold-off valve:


and that this valve is also listed as a hold-off valve (the assembly manual calls this a pressure regulator valve):


What the heck?! There's so much mis-information out there, please help me identify these with the correct terms in my writing.

I also find it very funny that the 1968 assembly manual has a JL8 page, but they weren't available as an RPO in '68.

Thanks,
Tony Huntimer
Camaro Performers Magazine

[m22mike]

One reason there is so much confusion about these two valves, is that most all the vendors don't know what they are.. ...and call them the wrong thing.
The round valve is a hold off valve. Holds off the front brakes untill around 40 PSI (I think ) so car will not nose dive.
The other valve of the fram rail is a press reg valve.
John Z posted some good info some time ago on the correct nomenclature and function on the 69 Brake valves. Later I will search for it . It seems the CRG message board is down right now.

Mike

[lzdick]

Tony,
The round valve (below the master cylinder) is usually referred to as a 'proportioning valve'...and as Mike said, keeps the car from nose diving.

FYI. The one pictured would need the large hex-nut on the front plated zinc (silver) to be correct.

[JohnZ]

The round one is the "metering" or "hold-off" valve, which denies fluid pressure to the front system until it sees 30-40 psi (to minimize "nosedive" at low speed, and to avoid behavior disconcerting to drivers while stopped on snow and ice). The one on the subframe below the driver's door is called a "pressure regulator valve" in the Chassis Service Manual, and is actually a true proportioning valve for the rear drum system.

The rectangular one directly below the master cylinder (which vendors usually call a "proportioning valve") is the distribution block, which contains the brake fluid differential pressure warning switch, which has no valving functions at all - it just illuminates the "Brake" warning light if there's a differential in fluid pressure between the front and rear systems.

In the mid-70's and later, all four functions (distribution, front hold-off, rear proportioning, and the warning light switch) were integrated into a single "combination valve".

Most vendors call them by the wrong name, and don't know what they do, either.

[RamAirDave]

JohnZ definitely knows the function and terminology on the parts.

The usage of the valve on the subframe is inconsistent.