Spongy Brakes - Master Cylinder?

[45Kevin]

My brakes need to be pumped up two or three times before they grab and then the pressure dissipates and the pedal sinks to the floor. When they are pumped up they work well.

I had discs from an '82 BJ43 installed on the 45 several years ago. They have always needed one pump of the pedal to get pressure. Once that one pump was made the brakes grabbed really well and held pressure as long as I was on the pedal.

The system does not seem to be loosing fluid so I am thinking this an issue with the M/C. Would a M/C from a post '80s fj40 fit on my booster?

I could get a rebuild kit for what I have now, but I don't think that would fix my original problem of needing one pump to pressurize the system.

This may be better posted in the 40s section, but I think the 45 has a larger bore M/C that the 40s.

Anyway, I'm just looking for advice before I rip into this. I could get a used or new 40s M/C fairly easy I think.

 

[Awl_TEQ]

Master cylinders and boosters are a real grey area for me. Is it possible that your original problem is a lack of volume in the output of the cylinder? Not a big enough piston meaning not enough fluid in one pump to grab the disc? Again, I know nuthin 'bout it.

I will need to edumacate myself as I want discs on four corners of my truck. I bought an OEM BJ42 cylinder but will it work?? What booster will work??


enough about me - somebody else chime in

 

[KillerPea]

Have you adjusted the rear drums?? Might want to snug those up before making any actual repairs.

 

[45Kevin]

Good idea, and I'll do that for sure. I still there are other issues though.

 

[40Flipper]

Mine's doing the same thing. I need to pump it once before it grabs. I have discs on all 4 corners. I bled the lines etc and it seems to be the MC. I have a spare one but I'm waiting for winter to put it in when I take it off the road. My mechanic was the one giving me a hand bleeding the brakes so I'm guessing his assessment is correct.

 

[45Kevin]

I have heard that disc brakes need to have a small amount of pressure always present at the wheel cylinders to keep the pads snug against the rotor, and drum brakes don't. I had always thought that the one pump I previously always needed was to snug up the pads.

I heard that the MC for the disc setup had a backflow valve do-hicky that kept a small amount of pressure at the wheel cylinder.

Are the 40 and 45 drum system MCs the same? If yes, then maybe a MC off a 40 disc setup would solve the problem. The disc swap must have been done by someone with success.

 

[tornadoalleycruiser]

also had one swap where the inner lining of the disk brake hose swelled when hitting pedal. might see how old the hoses are and think about replacing them.