Brakes Won't Hold Pressure...Again.

1972 Mustang coupe with manual drums, new wheel cylinders, hoses and shoes.

I just installed my second master cylinder, as the first one would pump up to some pressure (after several pumps), but the pedal travel was extreme and as soon as foot pressure was released from the pedal all pressure in the system went away and the pedal would go to the floorboard. After some research reading every brake thread in this forum and a ton of stuff elsewhere, it seemed that I'd ruined a seal in the process of bleeding a completely dry brake system, except for the master cylinder, which was bled prior to installation. Oh, well.

Today, with new master cylinder in hand, I bled the master cylinder using my vacuum pump, performed the install and bled the four corners with the pump, as well. I didn't spare any fluid in the process and I can't imagine there's any air left in the lines at this point.

Now the pedal will firm up after a single pump and hold pressure until foot pressure is released. The pedal travel is just right, too, which is great. However, when foot pressure is released the system will lose pressure and require a pump of the foot in order to regain firmness, though the pedal has more resistance now than with the old master cylinder when the system loses pressure. Otherwise, I'm experiencing the same problem as with the old master cylinder.

So, what am I missing here? Can anyone suggest a course of action to remedy this?

Assuming you have rear drum brakes, are they adjusted properly? If they are not, they will require more fluid to expand the wheel cylinders and make contact with the drums and will behave as you are describing.

Also, are you using DOT 3 (non-silicone) brake fluid?

some ideas.

have you adjusted all 4 drums?

brake distribution block issue?

problem with pedal ratio on pedal assembly.

some ideas.

have you adjusted all 4 drums?

brake distribution block issue?

problem with pedal ratio on pedal assembly.

Click to expand...

Good point! I don't know why I assumed he had discs up front....

+1 on the adjustments. Also make sure that the drums are within roundness limits and not worn beyond the wear limits (maximum diameter). Make sure nothing is preventing the pedal from returning all of the way and it isn't keeping the push rod and piston from fully releasing.

I appreciate all the responses. Though I did attempt to self-adjust the brakes, I'll start with the adjustment issue. I hope that's the fix. I'll be sure to mention how things go tomorrow.

air in the lines.

I was surprise how much air was in my brake system

when I did a total flush using a clear tube to see the air!!!

Paul

mine has a small leak in one of the wheel cylinders which causes it to take in air ad cause a spongy feeling to the brake pedal after a while.

air in the lines.

I was surprise how much air was in my brake system

when I did a total flush using a clear tube to see the air!!!

Paul

Click to expand...

The first run with empty lines and wheel cylinders obviously had a lot of air, more than the vacuum pump could tackle. Using the brake pedal was the only way I got could going anywhere. Today was pretty easy, though I used about a quart of brake fluid just making sure.

mine has a small leak in one of the wheel cylinders which causes it to take in air ad cause a spongy feeling to the brake pedal after a while.

Click to expand...

I'm really not in the mood to chase issues like that. Hopefully the adjustment works and I'm on the road tomorrow.

Just a question. Do you remember what diameter your master cylinder bore was? The drum brake and caliper brake master cylinders are different dia. The drum is I think 1" and the caliper maybe a 15/16". Don't know if that would make any difference.

Like others have said run the shoes out until they drag a little and back off some. You will have to keep hitting the brake pedal while adjusting to keep the shoes centered. Before they got the self adjusters it was quite a process to get them adjusted. I know we would adjust and take out and hit the brakes hard and if it pulled to right you would back off on the right just a little until you got straight line braking. After the self adjusters you just had to back up several times and slam on the brakes to make them adjust. Sometimes the self adjusters don't work right due to wear.

David

Just a question. Do you remember what diameter your master cylinder bore was? The drum brake and caliper brake master cylinders are different dia. The drum is I think 1" and the caliper maybe a 15/16". Don't know if that would make any difference.

Like others have said run the shoes out until they drag a little and back off some. You will have to keep hitting the brake pedal while adjusting to keep the shoes centered. Before they got the self adjusters it was quite a process to get them adjusted. I know we would adjust and take out and hit the brakes hard and if it pulled to right you would back off on the right just a little until you got straight line braking. After the self adjusters you just had to back up several times and slam on the brakes to make them adjust. Sometimes the self adjusters don't work right due to wear.

David

Click to expand...

As expected, the technical knowledge and advice here is great. However, the solution was to adjust the brakes as you describe above. An ass pain out in the cold, but productive.

The front brakes were about right. Minor back and forth got me settled. The rear brakes were completely out of adjustment and took a lot of attention, but the pedal travel, feel of the pedal and pressure being maintained in the system are all just right.

Thank you very much. I'm putting the car back on the road today after more than a dozen years.

one thing to keep in mind.

if you changed the drums and the shoes they take a while to bed correctly usually the shoes curve doesn't match the drums curve and they say you need a couple 100 miles before things settle they also talk about heat cycles that can change the way the drums and shoes bed to each other.

at any rate, what you should do is find a safe empty area and do a hard stop from 30mph.. get out of the car and look at the tire marks you need to see all 4 marks leaving rubber to confirm all 4 drums are at least set up and locking the tires well. next take note of how the car stopped, do you need to correct the steering to keep it straight etc. if the car is pulling left then you will want to add a click or two to the adjusters on the right side of the car. the rears are usually responsible for keeping you straight. so you add a click or 2 to the star adjuster to make that side drag a little more. again as the brakes bed you may need to change the star adjusters again. so keep an eye on things for the first 500 miles.

now an area that is overlooked with drums is the brake backing plates has 3 landings per side 6 total that the shoe rests on. These wear out and gouge, people forget to grease them and they should be welded up and repaired if damage is found. the worn pad landings, make the shoes hang up, bind, not float correctly and can cause a lock up on one side when heavy stopping causing erratic braking.

so those pad landings need to be flat and level, gouges need to be welded up and grinded down flat.

here is a photo showing the problem



so when working on drums, remember to inspect your backing plates fix them as needed and then regrease those pads before installing the shoes

one thing to keep in mind.

if you changed the drums and the shoes they take a while to bed correctly usually the shoes curve doesn't match the drums curve and they say you need a couple 100 miles before things settle they also talk about heat cycles that can change the way the drums and shoes bed to each other.

at any rate, what you should do is find a safe empty area and do a hard stop from 30mph.. get out of the car and look at the tire marks you need to see all 4 marks leaving rubber to confirm all 4 drums are at least set up and locking the tires well. next take note of how the car stopped, do you need to correct the steering to keep it straight etc. if the car is pulling left then you will want to add a click or two to the adjusters on the right side of the car. the rears are usually responsible for keeping you straight. so you add a click or 2 to the star adjuster to make that side drag a little more. again as the brakes bed you may need to change the star adjusters again. so keep an eye on things for the first 500 miles.

now an area that is overlooked with drums is the brake backing plates has 3 landings per side 6 total that the shoe rests on. These wear out and gouge, people forget to grease them and they should be welded up and repaired if damage is found. the worn pad landings, make the shoes hang up, bind, not float correctly and can cause a lock up on one side when heavy stopping causing erratic braking.

so those pad landings need to be flat and level, gouges need to be welded up and grinded down flat.

here is a photo showing the problem

so when working on drums, remember to inspect your backing plates fix them as needed and then regrease those pads before installing the shoes

Click to expand...

Nice information. I'll be keeping a close eye on it as things break in, though everything was great driving around today. The pad landings are good, with the car having only 55k miles and the 9" out back only a fraction of that.

It's been really go just having the time to work on it, at last.

Glad you got your brake problem solved easily

I recently rebuilt my brake system...had to weld up the pads and grind them back down.

Glad you got your brake problem solved easily

I recently rebuilt my brake system...had to weld up the pads and grind them back down.

Click to expand...

Nice. Yours look...better...than mine. Mine will be replaced with discs later, so they can take the ugly with them.

In the past when I have put a small amount of grease on those points (Copaslip) upon examining later some of it has migrated into / seeped through the shoe material.

Years ago I had a major issue with an old Mini. I manually adjusted the drums resulting in them overheating & being warped to a conical shape. Of course I did not realise this & kept trying to bleed the brakes which exhibited that 'one pump' required to get good scenario. Eventually I put G clamps on the wheel cylinders as a test and voila, rock solid pedal! New drums sourced after that.