New rear brake lines///Not bleeding

Jan 8, 2022
Changed my brake lines due to leak not my first rodeo did this on my c10 when I converted to disc brakes and again when a line broke but for some reason can’t get rear brakes to bleed. Should I change proportioning valve???


  • D8A39287-2553-4F48-AB95-D9B8A0784C9D.jpeg
    2.2 MB · Views: 14


Jan 19, 2021
There should be a button on your proportioning valve you can press to reset it. Sounds like it is working like it should when there is a total loss of pressure to the rear. Also on mine when I replaced the rear brake line I ended up having to use a pressure bleeder. Good luck with it. It took me a several tries resetting the prop valve before it finally let fluid to the rear line.
  • Like
Reactions: 1 user


Supporting Member
Jan 2, 2006
A word about the "reset button" for the stock combination valve on a G-body....there isn't one. It's a delay valve.


The "button" on a G-body combination valve doesn't do anything for the rear brakes. Nor does it do anything for "resetting". It's actually just a rubber dust cap. You take it off and can install a special tool to hold the delay valve open which help you bleed the FRONT brakes only. The little valve briefly delays pressure to the disc brake side to allow the rear brakes to go first. The pin is held OUT by the little special tool to make bleeding easier. It's not absolutely necessary to use the special holding tool, but it just makes it easier. Especially if you THINK you're going to gravity drain the front brakes. Cuz then it won't. Pushing on it will only at worst bend the stem of an already closed valve.

You likely have actuated the sliding pin, or shuttle valve, inside the valve which if there's a large D/P across the valve, which indicates to the valve that one side is leaking, the shuttle valve moves to close off the low pressure side, preserving the "good half" of the valve, allowing you to still have front brakes, in your case, since the rear was leaking. When that happens, the switch pin is grounded on your combination valve wire and lights up your brake light indicating you've got brake trouble.

Your job is to re-center the shuttle valve. Resetting it can be attempted hydraulically (see videos below), or if you can somehow take the switch off the top and see to get a tiny jewler's screwdriver or pointy straight pick tool in there to get the shuttle valve to center is all you would need.

The key when bleeding is ensuring the shuttle valve is centered, and use the "bleeder tool" in place of the switch to lock the shuttle valve in place. If your switch is wet with fluid when you remove it, that means your shuttle valve seals are leaking and you need to replace the combination valve. Unless you can rebuild it (unlikely because it's a real PITA).

Here's a couple of videos that may help you understand better this weird combination valve. The first one is pretty good explaining what things do. Good luck.

  • Like
  • Informative
Reactions: 4 users


Royal Smart Person
Jun 6, 2014
Houston, TX.
Excellent post! Thanks for taking the time. (y)(y)
  • Agree
Reactions: 1 users


Jan 19, 2021
Fantastic post Geezer! Great info! I had issues with getting fluid past the prop valve after replacing rear brake lines on my Monte that sat 15 years with no rear brakes, then I asked my dad (71 year old retired mechanic) and helped me get it done. He did all the hard work while I stood aside lol. He said he reset the prop valve so that’s where I got that. I had no idea it was so complicated. Anyway nice post and I hope the OP gets his brakes fixed

GBodyForum is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon, the Amazon logo, AmazonSupply, and the AmazonSupply logo are trademarks of, Inc. or its affiliates.

Please support GBodyForum Sponsors

Classic Truck ConsolesDixie Restoration DepotMike's MontesP-S-TSouthside Machine PerformanceUMI Performance

ContactAdmin@GBodyForum.comfor info on becoming a sponsor