I bought my 1968 base model 6 months ago, it has power front disc brakes and came with a new master cylinder and calipers. I replaced the rear brake cylinders and vacuum booster then bled the whole system and have always experienced front brake drag. I’ve been reading threads with others that had similar issues and have since then replaced the front flex hoses and brake distribution block.
After applying the brake pedal the discs are practically locked up until I open one of the bleeders and it releases. I’ve also loosened the master cylinder from the booster and the brakes are still locked up so it’s not the rod adjustment. I can’t figure out why the fluid stays under pressure after stepping on the brakes, anyone have any other ideas?
You might have either an incorrect master cylinder or it was mistakenly assembled with a residual pressure valve in the front brake port. Simple test is pump the brakes up and then crack the line for the front brakes at the master. If fluid squirts out and the brakes release, you’ve found your problem.
The rod going from the booster to the master cylinder will cause this if it’s adjusted too long. There is also a failure mode of the booster that will cause this.
I think your booster rod is too long as well. I put a dab of yellow paint on the tip of the rod and put the master cylinder on the booster and if it went flush to the booster without any pressure and the yellow paint transferred to the master cylinder piston, I called it good. Brakes don’t stick. For what it’s worth.
The rod is adjusted perfectly, as I mentioned in the first post I loosened the master cylinder and had 1/4” gap between the booster and they were still locked up. I think the previous owner put the wrong master cylinder since Hemikiller mentioned that the residual pressure valve may be the issue. I pulled the master and removed the RPV and now the pedal goes almost all the way to the floor before it’s starts to brake.
I’ve read that the front disc need 2 psi of residual pressure while drums require 10 psi, that would explain why the front was staying pressurized. Going to order the correct master and try again.
These $12 tools from Amazon are great for adjusting the booster rod
Residual pressure valves, commonly built into OEM master cylinders, maintain light pressure (2-psi for disc, 10-psi for drum) in brake lines to prevent fluid drain-back, reduce pedal travel, and ensure fast brake engagement. They are typically found under the seat fittings, consisting of a rubber check valve and spring, and often need removal when converting from drum to disc.
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Key Aspects of Built-in Residual Valves
Function: These valves prevent air from entering the system and maintain 2-psi to 10-psi, preventing a “spongy” or low brake pedal.
Location: They are located directly inside the master cylinder outlet ports, often under the seat where the brake line flare nut connects.
Identification: In many older or drum-specific master cylinders, a 10-psi valve is standard to keep pressure against drum brake return springs.
Removal/Conversion: If converting rear drum brakes to disc brakes, the factory 10-psi residual valve must be removed from the master cylinder to prevent constant brake drag and overheating. This is often done by using a screw or hook to pull the rubber/brass assembly out.
Testing: You can check for their presence by looking into the port or gently poking with a small tool to feel for the rubber check valve.
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For custom or modified systems, additional external valves (2 lb for discs, 10 lb for drums) may be required if the master cylinder is mounted lower than the calipers, such as under the floor,.
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Disc Brakes: Use 2-psi valves to prevent calipers from allowing fluid to drain back.
Drum Brakes: Use 10-psi valves to keep brake shoes close to the drum.
The inside of a soft brake line can deteriorate and block up. That will restrict fluid flow and cause the brake to stay locked on. I have experienced the problem…
Yes, I screwed a small screw into the rear port for the front brakes and was able to remove the RPV. After bleeding the brakes (with the special screw to hold the proportioning piston) I drove the car and it grabbed really hard on the right front. Getting back in the air I noticed the left front was stick sticking a little.
I went to bleed the front again and the right side bled great, the left drivers side barely had any flow. I disconnected the brake line to that caliper from the brand new repro distribution valve and hardly anything comes out when pressing the brake.
I am totally perplexed now why the new proportioning valve or distribution block is not allowing pressure to the front left caliper
