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brakes balance on a 74' 620 4x4.


dimedriver

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Hi,

 

I have a 620 that was converted to 4wd sometime in the past. It is currently having braking problems. Mostly the balance is way off and the pedal is spongy. The rear wheels lock up 1st and have caused me to get quite sideways at one emergency stop recently. This is also one of the major factors keeping me from taking the truck on the interstate.

 

 

The PO said he replaced the master cylinder. It is nice and shinny so I believe him. Its also a 3/4 inch master cylinder. but he may not have bleed it right. Im going to bench bleed it this weekend and flush out the whole system because the fluid turned black pretty fast after bleed it when I 1st got the truck.

 

I have done a little research. Im pretty sure the front axle is a Dana 25 and Redeye agrees from seeing my pictures in an older post. These had 1 inch or 1.25 inch wheel cylinders with 9 or 11 inch drums depending on year and type of truck they were under. The 620 had 3/4 inch all the way around unless did not have power brakes or it was that funny looking half car truck thing.

 

Im guessing 1 inch up front and 3/4 in the back is the problem. Was there some strange master cylinder that should be on this truck to compensate for this? Or would there have been other mods to make the braking balance better?

 

I have found a few threads mentioning some pressure valve under the passenger side that needs to be bleed along with the brakes but I could not find it under my truck. Its pretty easy to look its a 4x4 with some lift. I also looked in the FSM on the 620tech page and it did not mention this valve in the bleed procedure.

 

Any help would be greatly appreciated.

 

-Dimedriver

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first bleed everything, even with the adjustable bias valve not there, it should still have good pedal feel. youll most likely find air in the master. then if that doesnt work, the add a manual adjust bias valve. this is what i had to do, i just replaced the stock unit with the manual one under the pass seat, mine was faulty and not limiting anything. my stopping is amazing now. as far as the wheel cyl. you need them to be larger than the master. i thought it was the other way round, but was straightened out by a nice datsun guy once. lol

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