so i have recently redone all the braked on my truck. all new pads new rotors on the the front, replaced the master cylinder. once i got everything put back together my brakes wont hold pressure. it will hold when it is off but as soon as a start it it goes to the floor. and i only get breaks when i pump them several times. any one ever had a problem like this any help is really appreciated.

 

It does sound like you have lots of air in the hydraulic system. Did you bleed the air from the system and never allowed the level in the master cyl to go below the half full level ? Did you bleed from the wheel furthest from the master cyl first then work the next furthest and so on? This will take a lot of brake fluid to do correctly, so if you don't have at least a quart to do it properly,get more.
If the bleeder fittings on the calipers and wheel cyls are frozen and you cannot open them without wringing them off , it gets complicated.
Sometimes, theoretically, air rises in the system. Maybe you might be able to let some of it out by loosening the lines at the master cyl. Did you "bench bleed" the master cyl before installing and connecting the lines?

 

i have bleed almost a quart and a half through it and i did do it correctly that way im so confused about it

 

If the rear brakes are in need of adjustment it can add to the low pedal problem. They have self adjusters, but they freeze up and need to be manually adjusted.
If you have a questionable master cyl, by the way was the master a new or rebuilt unit?
Many times the replacements are defective, new=never ever worked !
If the only time you opened the hydraulic system was to replace the master , depending on how much air got into the system , it can be a stinker to get all that air out . Just opening the bleeder once and getting all fluid and no air bubbles doesn't mean all the air is out of the line to that wheel. You have to do it several times per wheel because the air has to come from the master cyl all the way to the individual wheel to get it out of there.

 

You know, sometimes on this forum I feel like I'm about 150 years olds when I see Hanky recommend a rear brake adjustment (which is SOP). Does anyone besides me remember when a low brake pedal was caused by out of adjustment FRONT drum brakes?
I'll bet you still own a TIMING LIGHT, Hanky (just like me!)

 

Hi Greasemark,
Glad to hear from you.
Yes , still have a timing light and a magnetic timing probe too, remember those ?
Miss your good trans advice, stay well !

 

I've had cars that seem like they take forever to bleed, even if I do them "as recommended" the first time, keep bleeding!