hey all,
My 71 is having some brake problems. The truck is still running the 4 wheel drum brakes, and I would like to see if i can get them working correctly. I have recently replaced the front shoes and wheel cylinders. The problem is that when I bleed the brakes, after I am done and I get in the truck the brake petal is really firm and feels like it has great pressure, but when I start the engine the pedal immediately gets soft and spongy again like it felt before I bled the brakes. I am thinking that it has to be power booster related (somehow related to engine vacuum) but I don't know. I need some help, this im my daily driver and I am sick of it having weak brakes. Hope someone knows what's going on, I sure don't. Thanks Cole

 

if you have power brakes the pedal will feel harder when its not runnin. the engine vacum will make the pedal feel easyer. does it have a power booster on it?

 

yes the truck has a power booster. I went out and adjusted the drums to where there was light to medium drag on the wheels when I tried to spin them. This seemed to help a tiny bit, but there is still something wrong, it takes way to much effort on the brake pedal to get the truck to stop. This is really becoming a pain in traffic and at stop lights. How much drag should be on the wheels when I adjust the drums? What else can I check? Thanks for the replies, keep them coming.

 

By chance, did you also adjust your back brakes? The back brakes play a big role in pedal function. Whenever I adjust brakes I almost always do all 4, if I have 4 wheel drum brakes. I need to replace my rear drum shoes so I didn't adjust the rears on my 1968 F100 last weekend because I was going to do the rears this weekend.

It's been a while since I had power brakes in a 60s vehicle. They weren't 4-wheel drum though. I know if the booster starts to leak vacuum the pressure that the internal parts of the booster apply to the master cylinder's push rod may vary. I don't want to steer you to a new brake booster because I'm not 100% sure it is the booster that is causing your problem.

When you apply your brakes when sitting and idling the truck at home can you hear any vacuum sounds coming from around the brake push rod up near the firewall? If a friend presses the brake pedal for you (in the driveway or vented garage) can you hear any vacuum leaking sounds near the booster / master cylinder connection under the hood? When you press the brake pedal does your idle get rough or change in any way? With the truck idling can you hear vacuum sounds coming from the pedal push rod area under the dash,or at the connection between the booster and master cylinder without pressing the brake pedal? If any of these four are a YES then you MIGHT have a bad brake booster.

I had a booster in my 1967 Mustang that had a seal stick between the master cylinder and booster. It started leaking vacuum. I corrected it at the time, but, I eventually replaced the pair. With the Mustangs the reman booster and reman master cylinder came as one unit that said seperating the pair voided the warranty. They were expensive though, from NAPA, I believe.

When adjusting your brakes the tire should spin approximately one full revolution to about one and a half turns, or that is the way I have always adjusted my drum brakes. The rears are a little more work because I have always had both tires turn when I am adjusting them which makes turning them to adjust the brakes difficult.

Another thing most people don't do when replacing their brake shoes is replace the springs and adjuster cables. I can't say from personal experience, but I have been told if you replace the springs and cables in the drums the brakes will keep themselves adjusted like they did when they were new. I'll know more about this in the months to come because that is what I did when I did my front brakes last weekend. I am going to do this with the rears too.

 

30 years ago I drove a 71 F100 with the same problem. I replaced the shoes, rebuilt all the wheel cylinders and still had some problems. I finally replaced the master cylinder. This fixed it. Don't know if this will help you but thought I would throw it out.

 

Yes, that too. If you go the route to replace the master cylinder be sure to bench-bleed the master cylinder before you install it on the truck.