I am preparing to replace the master cylinder and booster on my 68 Malibu. Can anyone tell me what the valve is that is mounted on the booster? One brake line that leaves the master cylinder is connected to it, then the line that leaves this valve runs down to a valve that is mounted on the frame (both brake lines run to the valve on the frame). Will I have to replace the valve that is mounted on the booster when I replace the booster?

Thanks in advance!!

The valve on your booster is the hold-off valve for your front discs. You don't have to replace it if it works. It's purpose is to delay the front discs from getting full pressure untill the rears have started applying. Without it, your brakes would be soft/low until pumped because the drums have to travel farther to apply than the discs.

The block on the frame is a distribution block. It does just what the name says.

If your master cylinder has stopped working or is full of crud more than likely the Prop/Hold off valve is full of junk as well. Best bet is to disassemble it. Wash it down with denatured alcohol and inspect it. Do not use any petroleum based cleaners as they will ruin the seals/rubber. If it's not severely pitted you may be able to reuse it. Or have it rebuilt. I believe Mickey hale does these 68-69. Ground Up as well Inline Tube sell replacements.

I'm going through my brake system now and found rust through out the system even though it was still working after 30 years. If your going to do it you may as well do it right.
Dave

Unless you are going for show stock look and number maching parts, there are a lot of great aftermarket unit you get for the 68. Complete new booster/master units with porportionong valve assemblies on the market. Chech out guys like summitt or jegs.

if you are going to reuse valve, put it in a vice to unscrew the big nut on the front.
be carful because there is a hevy duty spring inside, it will fly out. once apart just clean out. very simple design.
when you reinstall , there is a small round rod sticking out the back. you have to get the smallest c clamp to hold this rod in when bleeding the brakes. if you do not, the brakes will not bleed properly. pedal will always be soft.