[weekend warrior]

Question? I have a 72 c10 with an ls motor i recently converted the rear drum brakes to disc. Got a new master cylinder and booster plus a proportioning valve. Bleed the entire brake system. Pedal is firm but as soon as the engine comes on pedal goes all the way down. Does anyone know why ? Thanks in advance.

 

[BillPac]

Check the vacuum booster. If you have pedal with no engine vacuum, something is amiss with it.

 

[chasracer]

Did the truck have power brakes prior to this? If not you may want to look at the pedal rod into the booster, some setups require a different length rod.

 

[weekend warrior]

Check the vacuum booster. If you have pedal with no engine vacuum, something is amiss with it.

It’s the second booster and the second master cylinder

 

[weekend warrior]

Did the truck have power brakes prior to this? If not you may want to look at the pedal rod into the booster, some setups require a different length rod.

Thank you it is a new booster

 

[chasracer]

These were just replacement parts then? I'm a little stumped as to why the power booster would cause the brake pedal to drop like that. I have seen some issues where the proportioning valve will get caught to one side or the other and cause bleeding issues. Are you positive that you have all the air completely out of the system. A hard pedal could just be a pedal ratio issue and is helped along by the booster. Wish I had more info for you.

 

[weekend warrior]

These were just replacement parts then? I'm a little stumped as to why the power booster would cause the brake pedal to drop like that. I have seen some issues where the proportioning valve will get caught to one side or the other and cause bleeding issues. Are you positive that you have all the air completely out of the system. A hard pedal could just be a pedal ratio issue and is helped along by the booster. Wish I had more info for you.

Thank you I did see a video for that proportioning valve needing a special tool when bleeding the brakes. Is there any other way to bleed it without that tool

 

[chasracer]

Did you switch out the proportioning valve? They are different between disc/drum and disk/disk. I think you are talking about that piece that holds the valve in place? If you have a grinder, you can get a piece of brass and grind it down until you have a 1/4 long, 1/8" stem that will hold the valve.