brakes again

[Landi]

new to the site , finding it great ,I know brake questions have been flogged but I have 1 of my own . I have just rebuilt and fitted a disc front diff from a HJ47 into my 1977 FJ45 ,using my stock 1-1/8" master cylinder ,have removed residual valve from front port , added proportioning valve and all is working great except NO matter how hard I stomp on the pedal ,fronts will not lock up, pedal gets VERY HARD near the end .Would a smaller diameter MS give me more pressure to callipar with less pedal effort ? any advice would be appreciated .

 

[vk7ybi]

Yes.. Its all about the piston surface area ratios..

I have 4WDB on mine with the 1 1/8 master and I can lock them all up.. Near bust the seat doing it though..

 

[vk7ybi]

I should add that all my braking happens in what seems about an inch of pedal movement.. If your pedal is going all the way to the floor then you might need to adjust the rear brakes and check for air in the system.. I've had best results with gravity bleeding..

 

[Landi]

I have tested it on gravel and can lock fronts up but no chance on the seal, rebuilding and maintaining drums in the past i am somewhat of and expert at adjusting and bleeding brakes now and pedal not spongy at all ,pedal goes very hard at around 2-3 inchs down no way I could get it to the floor.I have read that less surface area of MS piston creats more pressure but less volume ,I have been unable to find someone with a HJ47 to see what size bore they run stock but I know HJ60 etc run a 7/8 MS .Hey vk7ybi what converion path did you take for the rear discs Ive being looking into it myself ,Cheers

 

[vk7ybi]

Yep, less surface area means less volume per stroke, but more pressure for same pedal effort, but at the same time more pedal travel for same braking.

My RDB conversion is done on a FF axle, so is probably different to what you will need to do..

This is similar to what I did.. Rear Full Floater Disk Brake Conversion

 

[Landi]

Mine FF rear and have checked this post out ,would fab mine own mounting bracket though, I might try the 1" MS I have sitting in the shed see if that makes a difference and leave the rear conversion till I get this right, Cheers.