Probably should have posted over on the other form but you guys have always been spot on in the past. Son's 92 F-350 4x4 w/7.3 IDI had a front caliper hanging up so he bought two rebuilts from Advance Auto. Took out for test and found that the pedal leaks down slowly to the floor under heavy push. Checked for leaks and have none so I figgure that the old (original) master cylinder got pushed past its old stopping point when bleeding the system and was now by passing, seen it happen before. Go back to Advance for rebuilt master cylinder and no change. pedal still goes slowly to the floor when pushed hard. Before I take it back off and tell them that the rebuild master is no good tell me if there is another thing that could cause the pedal to slowly go to the floor (like maybe 10 seconds of push). I cant think of anything other than a leak or bad master cylinder

 

Well try plugging both ports on the master and seing if it bleeds down that will tell you if its the master.

 

The pedal shouldn't go to the floor. I assume that you did get all the air out. I don't say that being a smart a$$ but I have seen knowledgeable people have air troubles. I have before and I work on things for a living.

When I bleed a system now I always gravity bleed then when the pedal is pushed down I do NOT pump the pedal. Push it down one time and open the bleeder. Keep doing that. I have seen people pump the pedal when bleeding and they aireate the fluid and can't get ride of a soft pedal.

Now that the air is out of the system. The master cylinder should hold without dropping. That is with the truck off.

You didn't mention the back brakes. Are they ok? Your problem could be there. I ran into a brake problem a few years ago that was something like yours. Come to find out one of the back drums was cracked and it would swell and let the pedal go down. Not that common but can happen.

You could also have junk parts. It seems that more and more parts now days are junk. We are having more of the china junk coming into this country and it ends up at alot of parts stores.

Hopefully your calipers will be ok. Keep this in mind. If you have uneven pad wear, this is the rule of thumb. If it is the inner pad then your slides are hanging. If it is the outside pad then the piston is hanging. Junk caliper.

Oh one other thing. When you bench bleed a master cylinder never push the piston all the way in. Don't go over 3/4 of an inch. What happenes sometimes is when they are repaired the port that lets the fluid into the piston bore will have a bur left on it and when you bench bleed and push the piston all the way in that bur can cut the piston seals. During driving you don't push the piston that far in.

 

could be some air still in the brake lines. may try bleeding them again just to make sure before springing for another master. farthest brake from m/c first, then next farthest, etc. may or may not help, but worth checking.

speedrdr

 

I bought a master cylinder at Advance before.
The brakes were worse after 20 miles of driving than they were with the old master cylinder.

Then one of the local mechanics showed me why.
I got another master cylinder from Advance, and went by his shop.

He filled the cylinder with brake fluid, then pumped it all out into a paper cup.

I was blown away by how much metal cuttings were in the cup.
Evidently they are not cleaning out the master cylinder housing after the machining process.
All the cuttings lay there and cut the seals.
Then you have the problem you are having, pedal sinks to the floor and no fluid leaking under the truck.

After we flushed all the filings out, that same master cylinder is still on my truck, probably three years so far and works fine.

 

Does it have RABS? A faulty RABS valve unit can cause what you describe.

 

Yes, the truck has rear only ABS. Re bled the system and the valve under the cab. No air bubbles and no changes, same problem. Guess we will exchange the master cylinder and be carefull how we bleed the new one. Have heard that the ABS valve can cause the problem as can the brake booster(that one woulld require some explanation??) Just sems od tha either would suddenly be problematic whe we changed the calipers?

 

Just solved a mystery for me on that, glad quality is such a high priority. Wish I had asked that question before instead of slow torturous insanity. I guess we just take it at face value that it was done properly when it was built. Learn something new everyday.

 

Locally we had a run of master cylinders, wheel cylinders, slave cylinders and even calipers that were full of filings.

Advance Auto, Autozone, NAPA, Fisher/Federated Auto and the dealers all had the same garbage.

I think they all come from the same place, just in different boxes.

After about the third master cylinder in six months I was really getting fed up.
My mechanic friend calls one night, his computer crashed, can I come fix it.
I'll be right over after I change my master cylinder.
He say's come fix my computer, I'll fix the master cylinder if you get a new one.
Cool, I'll be right over, I have one from Advance.
So I go over and start on the computer and in a little he says I gotta come look at something.

That was when he filled the brand new mastercylinder with fluid, then pumped it all out into a paper cup.
The entire bottom of the cup, about a 16 oz cup was covered with metal shavings.
No white to be seen on the bottom at all.

Quality control .....I guess they were all on vacation or something.

That same master cylinder is still on my truck, I know it has been at least 3 years.
Before that I was getting less than 2 months out of master cylinders.
Clutch master cylinders and slave cylinders were the same way.

Now I will not buy a part that does not have a lifetime warranty.
Even if it doubles the price.

 

i've bench bled everything i've put on the truck that has fluid running through it. does this help remedy the problem of poor quality control/machining trash left in parts?

speedrdr

 

Speed, If you can get the the bore cleaned out without cutting the piston seals up then it will definetly help.

The bad part is that any metal that is left will rust up and get into other parts later. The bad thing about brake fluid it absorbs water so fast and easy that it is impossible to keep moisture out of it. DOT 5 is better as far that goes but you can't use it in ABS systems.

 

I wish there were mechanics like that around here.

Everybody does outsourcing these days. I have seen the same house brand hand tool under 3 different names, in three different stores. The only difference was the name, and the color on the handle plastic. All made in the same overseas factory. Same with parts like this, a brand name is worthless without a garantee. All you are really paying for is the name, and most parts companies could probably be run out of a laptop computer instead of a real factory. I guess you could go for the bottom or mid range price and get the exact same product, or pay through the nose for the top of the line and basically buy a waranty for the same thing. I'm glad my brakes are still good, for now at least.