Spongy Brake Question

[colnago]

Long story short, I did the drum-to-disc conversion recently, but I screwed up the driver caliper. I knew it was leaking, and probably bringing air into the system, but the brakes worked well enough to let me drive while I ordered a new caliper. Today, the pedal went to the floor,so it became my "fix the brakes" day by necessity.

So, I swapped calipers, and started bleeding. The first third of the pedal movement, there was hardly any resistance. I've never had this before. Is this due to air up at the MC? Should I do a bench bleed again? Once the brakes engage, the truck stops great. But with the first third having no action, the pedal is nearly to the floor for an emergency stop. Any advice?

Joseph

[PetesPonies]

There are two things that causes this. First would be air, second would be a internally leaking master cylinder. many times, people who are not very experienced, don't get all air bled out of the system. Sometimes, calipers are put on the wrong side, which can make for a spot for air to get trapped. So without being there, I can only tell you to double check and then bleed some more. If you are sure of the things I mentioned, then consider a faulty MC, with an internal leak.

[colnago]

Sometimes, calipers are put on the wrong side, which can make for a spot for air to get trapped.

Interesting comment. I found that in order to fit on the driver side, I had to order the caliper for the passenger side. Otherwise, I had one of the following problems:

1. The caliper had to be installed behind the wheel hub, which it didn’t do on the Bumps, or

2. The fluid intake and bleeder would be mounted on the bottom, so I would never be able to get the air out. This also didn't happen on the bumps.

I'm 99% sure that the caliper is installed correctly. I'm just very surprised that I instantly had no brakes, and equally surprised that the first third of the pedal doesn't do anything. Everything was fine last night. The only thing that I can think is that the bad caliper let air in the system, and that eventually worked its way to the MC.

Joseph

[ddcrim]

I would also suspect that the plunger inside the master cylinder is not returning all the way back. I have had this happen
when the plunger is pushed all the way in, like in your case. It will jam 1/2 way in and the brakes will act just like this.

[colnago]

Is there a way to un-stick it, or should I get a new MC?

Joseph

[ddcrim]

Check the plunger by removing the MC. You will have to undo the lines, but at this point you have to re-bleed anyway.
Push in the MC plunger with an appropriate tool (screwdriver). The plunger should push in and return all the way out
to the C clip stop on its own, a smooth action. If not, a MC rebuild kit is in order. Check the bore first. If rusted it will
have to be cleaned up. PS be careful pushing in the plunger as brake fluid will squirt out the connection holes. You may
be able to get it pulled back far enough with the lines connected.

[My427stang]

Sometimes when you bleed there is nothing for resistance until you get some air out, did it come up as you bled? Did you do both sides?

Also, when doing it that way, be sure to check the level in the m/c as soon as you start to see fluid, it'll empty real quick as it fills the caliper and lines.

[colnago]

Well, I pulled the MC off this morning. The primary piston was already at its fullest point. I took the opportunity to redo the bench bleed. Glad I did, because I got some air bubbles out. Guess I did a sub-par the first go-around. At this point, I'm pretty sure the problem isn't the MC.

This is a '68 MC on a '67 booster, so I had to buy them separately. During the install, I had to adjust the booster output shaft to be against the MC primary piston. I checked this again, and it looked good. So I bolted it all together, bled the system, and went for a drive. The pedal still has a couple of inches of travel before engaging, but it stops better now (probably because I did the bench bleed again). I checked the linkage under the dash, and everything looked good. The play is there whether the engine is running or not.

So, either I have a bad booster, or the booster output shaft isn't properly adjusted. Either way, I don't know why it was fine one day, and wonky the next. This is a rebuilt Cardone booster, by the way. I'll still be working this issue, but I can at least drive the beast to work while I'm figuring it out.

Joseph

[cep62]

So, either I have a bad booster, or the booster output shaft isn't properly adjusted. Either way, I don't know why it was fine one day, and wonky the next.

Another thing you can do is plug off the master cyl and try it.
if you have a good pedal every time , you will know thats not your problem.

[ultraranger]

The tip of the booster output rod should not be touching the back of the MC primary piston when the brake pedal is in the static position (brakes not being applied). There should be a .005"-.010" gap between the rod and the MC piston.

Simple procedure for adjusting the booster output rod to the MC.

https://www.google.com/url?sa=t&source= ... FR5qtkmUvg

[colnago]

Well, the bottom line is that the output rod from the booster wasn't adjusted long enough (or lost its adjustment over time). It ended up being about 0.2" short, which made an incredible difference on the input. So, I adjusted the rod, bled the system, took it out for a ride, and ... stopped! Now, I'm at the point where I hoped to be several weeks ago. Now I have a working system, and can start looking for leaks, decreased performance, etc. What a difference it is having working brakes.

Joseph

[ultraranger]

The booster output rod adjusted too short will cause a long brake pedal travel. The booster output rod adjusted too far out will keep the brakes partially applied even when the brake pedal isn't being pushed. This will cause the brakes to drag, get hot and can cause them to lock up.

[colnago]

It's definitely not too long. I could have probably made it a little longer, but I was afraid that there wouldn't be enough thread overlap left in the booster piston. I have lovingly responsive brakes now, so I figure that I can adjust from here. All it takes is time, patience, and brake fluid.

Joseph