ok, some background:
i replaced the master cylinder, most of the hard lines, the wheel cylinders are a few years old, as are the calipers. pads, shoes, rotors, and drums all look fine. brakes bleed fine. they stop fine. good vacuum at booster, with no noticeable leaks. pedal has plenty of pressure, and it doesn't go to the floor, except:

here's the problem. if i hit the pedal normally, everything is fine. the problem arises when i press on the pedal extremely lightly and slowly, and if i keep pushing it slowly and lightly, it will go almost to the floor before it actually starts applying the brakes. push it hard, or fast, as in a normal braking situation, then it behaves properly.

any suggestions? I tried searching here, on pirate, and on google, and cant seem to find someone with the exact problem I am seeing. any help, suggestions, links, or comments are appreciated.

thanks,
dewitt

 

You said that you have already installed a MC. Was it new or remanufactured? I am asking because it sounds like it is bypassing internally to me.

 

I am not 100% sure if it was reman or new. I would guess reman though, i got it at advance. so you think that slow pedal push it could bypass internal, but on a hard press, hold pressure? looks like i may be trying a new mc then.....

 

That really sounds like a defective MC.

 

Well, it is just a guess at this point, so take it for what it's worth. I have seen them with sympoms very similar to what you are describing, and a new MC fixed it.

Others may have other ideas, so maybe someone else will chime in with their opinion.

 

with constant light pressure, the brake fluid could seep around the pistons in the MC thus causing pressure to drop. a sudden hard press on the pedal doesn't give enough time for the pressure to leak around the pistons and also forces the piston seals harder against the bore.

 

sounds like another vote for the MC to me.....
what do you guys think about there just being air in the lines, probably wouldn't cause these symptoms, huh?
I think I am going to bleed them a bunch tonight, see what happens, and then invest in another MC if it stays the same.
oh, and for the record, the MC I put in was reman.

 

depending on how the mc was rebuilt, it's possible the bore was honed out and there wasn't an appropriate oversized piston/seal set available so they installed a standard size set which would be more prone to internal bypass/leaking. so, let us know how it goes.

 

i'd like to have some words with people that do the rebulding of things like that, i have such bad luck with reman stuff its not funny anymore. let me know if you guys think of anything else, i will keep you posted on how i make out.

 

well, i bought a new mc friday, put it in friday night, and bled everything, and it seems to have taken care of everything. Pedal is firmer than ever, and i can't seem to make it go to the floor by pushing it slowly. hopefully it stays that way.
thanks for your help guys
dewitt

 

good to hear.

 

I have had a similar problem when my back brakes were out of adjustment.