I have had my 78 since new. 302 Automatic, manual brakes. About two months ago, my master cylinder was leaking past, as the pedal would get mushy when setting at a light. I replaced the master cylinder all was well. Last week, I had her out and hit the brakes and normal pedal then I suddenly felt the brake drop suddenly near the floor, but with some limited braking. I checked the lines, found no leakage. I pulled the wheels, still no leakage. I thought maybe the master cylinder had failed. I took it off and took it to my parts house. They replaced it, I put it on, I had good pedal. The more I drove it the the less frim the pedal felt. I decided to do a relatively hard brake and again she went to the floor. This time stayed there. I got it back home and was sure I had blown a line or hose. I took the cap off the master cyclinder and too my surprise totally full. No drop in fluid at all. It takes me about three times to pump her up to get any braking and thats difficult because the pedal does not want to return. Any help would be greatly appreciated.

 

Sounds like you got another defective MC.

I went through two of them myself before I got a good one.

How is the Brake Fluid? If it isn't clear like it should, perhaps a brake fluid change would be wise.

Edit: How is the Brake Warning Lamp, does it come on and stay lit? Blink off when the brakes are pumped up? etc...

 

The brake fluid is clear. It seems unusal that I would go through two mc's. I had a hard time purging the air out on the bench. It took a long time with a lot of air.

 

I have no Brake warning light on the '78 that I know of. I know there is no sensor on the master cylinder. I could be on the equlizing valve?

 

I too went thru 2 MC this summer. I didn't have the time to do the work so had a local shop do it for me. A buddy of mine works there so I trust them not to screw stuff up or me. They did the first MC and it lasted a week. The 2nd one only last 2 days. 3rd one has been going good since July. They got the MCs from NAPA. I have had good luck for the most part with parts from NAPA, but not there MCs that week.

I am not a mechanic nor have I ever been accused of being one, but if you have power breaks could it be the break booster? Just a thought. If your pedal wont come back up then I guess I would start there, but that is just my dumb idea.

 

Are you sure you got a master for manual brakes and not power brakes? On some vehicles they are different ( I can't remember if this applies to these trucks, but I'm sure somebody will correct me if I'm wrong). I think the stroke lengths are different because the manual pedal has to have more leverage due to it not being power assisted. If you get the wrong one the pedal geometry might not be compatible and you may be either long stroking it or short stroking it. Or, as mentioned above, sometimes they just get a batch of crappy ones.

 

That is a great point.............I never even considered the possibility that they had given me the MC for power brakes. I can actually feel a bump in the pedals articulation(so to speak) when the pedal is near the end of the stroke. That could be where I am over-stroking it. The shaft was used from the old unit and I know it was right, it was the original.

I purchased from Autozone. Most there were born after the car was built and manual brakes may be somewhat confusing to them. I will check it out, thanks for the info.

 

The the dual brake warning switch is on the porportioning valve yes.

Purple/White Hash wire. Make sure it's not unplugged, or the lamp in the cluster isn't burned out.

If the porportioning valve isolates either the front or rear brakes due to a lack of pressure from the MC, the lamp is supposed to come on. Sometimes the valve gets stuck on one side or the other and needs to be replaced, and also the lamp switch itself that screws into the valve leaks and get defective as well.

 

Well I just spent the day in, on and under the 78. I took the master cylinder off and took it back to AutoZone. The master cylinder was a rebuilt manual brake master cylinder. I replaced it with a new manual cylinder, bench bled the unit. Installed and have absolutely the same results. No Brakes. The pedal will go to the floor and the pedal does not want to come back up. Additionally there is no drop in the fluid level in the master cylinder. If it went to the floor and I could find a leak or at least see a drop in the fluid level in the master cylinder then I would say there is a leak, I'm just not finding it. But why would the pedal not come back up? COuld I have an interference of brake rod to pedal? But why would I not have brakes. I need some help on this one. Any one have a picture of the brake pedal to master cylinder rod connection under the dash?

Appreciate any help.

 

Air In the system will keep the pedal from coming back up.

I didn't ask this before but, did you bleed the brake lines at the calipers, and wheel cylinders? Air might have gotten in the system.

The Porportioning valve might be stuck as well. The Job of the valve is to isolate the front or rear brakes when a loss of pressure is encountered. If It's stuck to one side, no brake fluid can reach the calipers or wheel cylinders. Did you get the Brake failure warning lamp working? It should flash on when you start the truck. The Lamp will go out if the valve is centered, it will remain lit if the valve is isolating the brake fluid from the rest of the system, front or rear.

 

I have not bleed the brakes at the calipers or at the cylinders. SInce I can find no obvious leaks and the fluid level has not dropped, I would not expect air to penetrate the closed system. So I didn't think air in the system was the problem But who knowa? The proportioning valve, I have no real experience with. Is there a way to check it? Is there a way to use a vacuum pump to check the individual circuits?

thanks for your help.....

 

That's the next thing I would do. Bleed the brakes at the calipers and cylinders. Air could have penetrated (and most likely did) when you had the MC lines disconnected at the MC. It doesn't take much air either.

There is no way really to check the porportioning valve, except with the warning lamp, or when you bleed your brakes you find no fluid coming from the MC.

 

I just checked with NAPA and AutoZone's on line parts search. No proportioning valve was available for my truck. If that is the problem, any idea where I can find one?

 

if you got rotors on the front and drums on the back you have a proportioning valve it could be bolted to the gear box or on driverside by the oil pan.

 

Yes, it's beside the oil pan on the drivers side. I was surprised to find that I could not buy a replacement for it at either Autozone or NAPA. I'm not sure how I will know if it is defective, but if I find it is.........Anyone know where I can find a replacement?