I've got a bit of a problem with the brakes on my 76 highboy. Everything except the lines and booster is about 2 months old. They worked great until the other day when I tried to stop and the pedal suddenly dropped to the floor. After 2-3 full pumps it'd build and hold good pressure. My first guess was that I had a front wheel bearing coming loose. Both were loose and were subsequently adjusted. I also re adjusted the rears while I was at it. Problem still exists but it's down to 1 pump and then a nice high and tight pedal. If I push it to the floor during the initial application the LR will lock up, but nothing else. No leaks, MC is full. The symptoms and sudden onset of the problem have me leaning towards a bad MC. The only piece that doesn't really fit is the lack of a falling pedal under pressure. I'm assuming that the cups are bypassing, but still seal well enough that they'll hold pressure once there's some back pressure to expand them. Anyone agree, disagree, have any other ideas? I'm open to suggestions. Thanks.

 

If your brakes are soft, you have air in the lines. No way around it. Do you see leaks anywhere? How do the brake hoses look? Somehow air got in the lines; the leak needs to be found, corrected, and the system re-bled.

 

No leaks anywhere. The MC has been full througout the process. I haven't had the system open since in I installed everything. The only way air could have migrated in is if it somehow was sucked in around the cups in the wheel cylinders or something. I've heard of it happening but haven't ever personally dealt with it. I'm going to try a re-bleed tonight to eliminate the possibility.

 

I would double check your rubber lines for cracks and other nasty abuse. Nothing worse than having what happened to you, except worse happen over a $100 set of brake lines not being replaced. You can buy some trick ones (set of 3 kevlar/ss jackets) for $100 that are DOT legal, and come in your choice of colors for the outer jacket.

I would not trust that master cylinder as far as I could throw it. Even if there was air in the lines still, your pedal should not out of the blue go to the floor then act normal 10 minutes later. Most remand master cylinders have a lifetime warranty from the national chain parts houses, warranty that thing!

Other things to inspect.. Do you have any mystery dampbess anywhere near your drums/front end? I assume you have front disc brakes? Either way, I would also inspect your calipers/and or wheel cylinders for any dampness at all, indicating bad seals.

Just because it is new, do not take anything for granted. I had an seat puckering experience when I brought home my crew cab. I tinkered with everything to make sure it would make the 50 mile drive home. I test drove the snot out of that thing making sure the brakes worked decently. I could only get one tire to lock up, but the brakes worked ok enough, so I made it a point to go slow, and not follow closely. The ebrake cable had seized in the housing, so no ebrakes. Well, about 5 miles into the trip, I notice the truck pulling badly to the drivers side. I stick my head out the window expecting to see a flat tire, but both on that side were good. Out of the corner of my eye, I see an Autozone, I go to slow down to make the turn to stop there. How great is it to have issues in front of a parts house BTW? Fortunately, when the brake pedal went to the floor, and no amount of pumping would give me brakes, I was able to down shift, make one full uninterrupted lap around the parking lot to bleed off speed, and one rolled over curb with the rear back tires locked up in park, was I able to stop the truck without wrecking it, or killing anybody. I thanked my lucky stars that nobody was around for me to kill.

The point I am trying to make is don't wonder if its going to happen, and don't take chances. Master cylinders and brake lines are cheap and easy, replacing a life isn't so cheap, or easy.

Another thing just occured to me, I do not see your profile filled out telling us whereabouts you live. If you live in a rust belt area, check all of your steel brake lines with a fine tooth comb. I have another story to tell about rusty brake lines. It was '97, had just bought an ex Sherrifs patrol car, 1990 Crown Brick with 180K on the clock. Was tooling around, things worked great in the car, rode nice, heat and AC worked, got scared looks from people whipping in behind people like I was inspecting license plates etc etc.... Well, one day, I made a panic stop, and the brakes "went to the floor" The car was still trying to stop, but the car was loosing brake pressure, requiring extendedf pumping. A 2am inspection on the side of the road with a flashlight told me that a rusty brake line had finally gotten thin enough to burst from the extra pressure and was leaking ever so slightly.. New steel lines throughout, the brakes worked cherry again.

 

I had a similar problem on my '74 F100. I replaced the MC with a reman from Napa. Got everything bleed out and working good. A week later I was actually pulling into Napa again and the pedal went to the floor. Did a quick pump and nice solid breaks. I drove it around a little more that day trying to duplicate the problem. It was intermittent. I finally just took the new MC off and took it back to Napa where they did a bench test on it and it came up defective. Not exactly sure what was wrong with it, but they handed me a new one. After putting the next new one on I haven't had a problem since.

So I would lean towards the MC, but I would double check the wheel cylinders for leaks as well.

 

I was in my local napa, they were giving a customer grief about possible incorrect installation of a master cylinder they sold him. The customer alleged that he did in fact install it correctly, and the brakes did work, except for one time. They either did not, or could not do any sort of bench testing of a MC (which I never knew a parts house would bother with. I stepped in with my big mouth and said to the manager, "Look, I strongly suggest you give this man another master cylinder so that when this one fails again, he wont rear end or T-bone the car that has your kids in it. Your and other peoples kids health and safety are not worth the grief you are giving this man over a $40 part." That ended the discussion right there, the guy got another master cylinder along with a lecture on how to properly bench bleed and install it.

Brakes are cheap.

Yeah it does suck for the corporate office that they are eating some money because some people do not know what they are doing, or are dishonest, but if they put a warranty on it, they either need to prove beyond a shadow of a doubt that their part is not the culprit, or just go the cheaper easier route and provide a replacement.

 

Switching to hydroboost is probably the best upgrade Ive ever done.

 

I have been thinking about doing this. Highboy brakes suck, even the 76-77 with disk brakes. Did you use a superduty setup or a chebby stuff? I like the idea of using a modern MC with modern propotioning systems. Sorry to butt in to this thread, but I had to ask.

 

The rear brake line from the proportioning valve back is new. The front lines look good. New calipers, pads, wheel cylinders, brake shoes, ect. No leaks anywhere, the MC is full, no drop in fluid level. After the first pump the brakes work great until you let off the pedal for a few seconds and then, back to the floor. I'm leaning towards the MC if only because of the sudden onset. The brakes were fine when I left the house and then, nothing. No leaks, no mystery spots on the driveway, no nothing. If it warms up enough I'll yank it and warranty it and see what I come up with. Thanks for all the help.

 

If the master cylinder is seeping, the fluid may be going into the booster, the reason why you don't see any brake fluid.

The low brake fluid warning light (plastic) switch that screws into the proportioning valve is notorious for leaking.

It developes minute cracks in the threads. Sooner or later this will cause the low brake fluid warning light on the dash to come one...and stay on.