Brake Problem

[xd2200]

Having trouble with brakes (all stock/DOT 3) on my M37. Got in it last week and pedal went 3/4 ways to the floor before braking. Used a pressure bleeder to remove air from the system and got a nice hard pedal (1" free play.) Truck sat for several days and the pedal went again 3/4 to the floor. Pressure bled again and got a hard pedal. Have repeated this cycle several times. There are no leaks and I am not losing any fluid. I suspect a bad master cylinder or perhaps a frozen wheel cylinder - any thoughts?

[Carter]

My first thought was that fluid is leaking past the master cyl's piston cup after it sits for a while resulting in the soft pedal if you are sure no fluid is leaking at the wheel cyls. and the rest of system is in good condition.

[m37jarhead]

Carter's got you on the right track. Most would agree your problem is probably fluid leaking past the internal seals of the master cylinder.
I've always said, anyone can make 'em go but stopping 'em is a lot more important.
Be safe and pull the master cylinder.
Keep us posted on your find.
JB

[xd2200]

Carter & m37jarhead - THANKS for your help - I will install a new master cylinder.

[rc51racer]

I'm having a similar problem, I'm going to install a plug in the output end of the master cylinder then pump the brakes to see how the pedal feels.

[ODUSAOK]

A number of years ago I had a problem like this on a WC-52 I do work on for a friend of mine.The brakes would lose pedal over time.The wheel cylinders were replaced with new units as two of them had minor leakage.The brake shoes were adjusted to specs.The problem improved but didn't go away so my friend asked me to install a new master cylinder he had bought.The problem persisted.We crawled under that truck many times in the next few years,saw no leaks,and just pumped the pedal a few times to bring it up full,and added some fluid very rarely.A real annoying situation,not to mention potentially dangerous.We did a clutch job and in the process of crawling in and out,I noticed moisture around the brake stop light switch.This had been changed to the new master cylinder a few years before,simply wiped of dirt and grime and reinstalled.I pulled it off the M/C,and it seemed to be fine.The moisture bothered me though,so I got it in better light,and there was a hairline crack in the bakelite around the poles of the switch.I was dumdfounded.I replaced the switch,bled the brakes,and sure enough,we haven't had a problem since.If you have a non-original switch that uses bakelite in it's construction,this could be a sourse of trouble.Regardless it taught me two things,one: you don't have to have a noticeable fluid leak(outbound) to have an air leak(inbound),and two:when you think you've done a thorough inspection,and a problem persists,you haven't really.
Chris

[pwrwagonfire]

Did you open the breather valve on the top of the MC? I was having a SLOWWWWWWW time bleeding mine until I read the service manual and saw mention of this valve

[xd2200]

As noted by Chris, I also noticed a loose pole on my brake light switch as I was replacing my master cylinder. Put everything back together and have a nice firm pedal. The failed switch is aftermarket as is the new one.

[rc51racer]

I crimped a short piece of brake line and connected it directly to the master cylinder, when I pressed the brake pedal it was hard as a rock.

Looking like a leaky brake cylinder now.