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Brake swap issue please help

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Hey, new to ratsun here as far as posting. Im having brake issues and wondering if anyone could point me in the right direction.


I installed Belltech lowering spindles for a hard body on my '79 620. I grabbed some front calipers, hubs and vented discs from a king cab at the junk yard (are those considered hard body?). Everything bolted up nicely so I went ahead and bled the brakes. They seemed squishy after a couple bleeding cycles. I decided to grab new rear wheel cylinders, a new master cylinder, bench bleed it and, after some research, bleed the NLSV with the rest of the system.


The brakes started to seem stiff until I turned the truck on. With the help of the brake booster, the pedal goes to the floor. Ive tested the booster's check valve and vacuum tested the booster itself. I can get the truck to stop but brakes are still feeling squishy.


There are only two possibilities that I can think of:


1. I may need a different master cylinder to match my front calipers.


2. My drums may need severe adjustment. (Even after reading my truck's manual I don't understand the adjusting procedure)

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Go get a master from a Late 70's-1980 Z car.

Repeat process.



Have drums adjusted tight before bleeding.

Not so tight that they can roll, but tight enough that you'll only be pushing air out of the lines when bleeding. Not moving shoes.

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The disc brake '78 and '79 620 had a 13/16" (0.8125") master. Going to D21 or even later 720 front calipers which are much larger, will require a larger master. The 720 uses a 15/16" and D21 use a 15/16" or 1" depending on application, so start looking there.


If all the air is out it will work but need to push more volume of fluid and so will travel farther and feel squishy.

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At the junkyard now and pulled a 15/16". There's a small amount of fluid coming from the "cup" where the pushrod goes in. Should that concern me?



Yes.  It needs a rebuild to correct the leakage unless the shaft is so worn that you need a complete rebuild kit including everything but the main casting.  And then you have to hope you can ream/polish the bore without going over bore for the replacement parts.  See what a pickle this is?

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Also, I thinks its the 78-79 Z (15/16) that is setup with disks up front and drums in the back. This is the master you need as it has the right residual valves.


79-80 is the 15/16"

Everything before that is the 7/8"


15/16" are hard to find new/freshly rebuilt. Hard to find a rebuild kit for them even.

But the 15/16" is the one you want.


RockAuto has a Cardone MC for less than $30+core right now.

Get it!

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Did you bleed the Load Sensing Valve.?


I personally don't think that the bigger MC is required, besides the 15/16" is exspensive and damn near impossible to find/rebuild locally.


I put D21 dropped spindles & twin piston calipers on my red 78' 620 & used the stock 13/16" MC with ZERO issues. The pedal is only slightly lower than stock.


On my green 78' 620 I'm going to use a 7/8" MC just to see how much it brings up the pedal.... only because I'm curious & it needs a new MC anyhow.     

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