Chevrolet Repair: brakes, brake fluid reservoir, dowel rod

Question
for the past few months and 10000 miles, i have been expiriencing brake problems. almost every time i would come to a complete stop from a low (5-10 mph) speed, the brake pedal would do one of two things. it would either travel all the way to the floorboard while maintaining the same brake pressure, or it would jolt back towards me, while maintaining pressure, and it would also make an unusuable whinning/groaning sound which could be heard over my CB. i replaced the booster and the master cylinder, and that got rid of the pedal coming back up and the unusual noise. but it didnt get rid of the pedal going all the way to the floor. every now and then, it will do this. i cant figure why it still does this. maybe it could be one of the calipers? i dont know. its got me confused and pissed off. the truck has been through some heavy duty mud in its life for the first 91000 miles. i havent been through any since the engine blew then and im at 108000 miles. maybe there could be mud inside one of the calipers or inside something else goofing it up. any help would be greatly appreciated. thank you

Answer
Hi Bryan,

Brake pedal slowly to the floor is master cylinder.  You changed that.  I have to ask if you blead the system correctly.  Did you bench bleed the master cylinder?  Did you bleed the brakes in proper order.  The low pedal 'sometimes' sounds like air.  Low pedal all the time is possible rear brakes out of adjustment.  

Bench bleed:  With the master cylinder in a vice, fill the master cylinder with brake fluid.  Then attach short brake lines to the bottom front and rear fitting holes.  Bend the small brake lines so they go from the front and rear holes and dip into the top of the master cylinder (lid is off).  Using a dowel rod, operate the master cylinder piston.  Go all the way through the travel.  Air will blow out the front and rear holes and into the brake fluid reservoir.  Keep pumping this piston until all air stops bubbling in the master cylinder reservoir.  That is it.  If you didn't do this, you master cylinder will not ever be able to bleed this air out.  

Brake bleed proper order:  Drums in rear, and disc in front:  furthest rear wheel from the master cylinder (passanger rear), then driver rear, then passenger front, then driver front.  Disc all around:  rear passanger, front driver, rear driver, front passanger.

You get your pedal height from the rear drum adjustment.  The tighter you have this adjustment, the higher the pedal.  If you have worn shoes and bad adjusters, your pedal could get low.  Not too tight though.  Just a slight drag is perfect.  

Low pedal also comes from low master cylinder level.  

Bad calipers are stiff braking, and strong pulling to one side upon braking.  

I suspect air in lines, may need brake shoes, or adjustment here.  The jolt you describe at slow speed braking, sounds like it could be pulsating pedal. This indicates warped front rotors.  May want to buy new ones.  I just did brakes on my 4x4 1993 Yukon and the rotors new were only $15 each.  

These issues are not caused by mud.  That would have washed out after a good rain.  If you get stumped again, write again.  

Good luck.

C J S