Hyper-sensitive brakes - 79 620, HB vented rotor conversion

[BarryA]

Brakes on the 79 620 KC have been a bit "grabby" since I got the truck, but since doing the HB vented rotor conversion, are hyper-sensitive.

 

Pedal travel seems a bit long, feels like the rear shoes contact the drums about 20-25% of the way into the pedal travel, but don't really get any braking force until the front calipers start to squeeze the rotors toward the bottom of the pedal travel. Once the brakes start to bite, it takes almost no pedal travel to get full braking force - difficult to modulate, and feels like you're going through the windshield.

 

System has been bled a couple of times and the pedal is firm, not "squishy", so I don't think it's an issue with air in the lines anywhere.

 

Stock 620 disc brake master cylinder is still installed, not sure if swapping to a larger bore 240Z unit will make a difference, or if the power brakes are just over-boosted - anybody else dealt with this situation?

[yello620]

I think that you might have a problem in the load sensing proportioning valve that is located on the passenger frame rail under the cab. I dont think that you have a master cylinder problem of air in the lines. If the valve is not working properly internally, then you will get problems like you described. Problem is that you cant really see the internals of them to prove the problem.

[Z-train]

Un hook the vacuum.THis sounds like a booster issue.

[yello620]

I have to disagree with unhooking the booster, It was engineered by nissan to be in that system and function. It is designed to help reduce your pedal effort. Not really any way that it can cause a sesitive pedal, because the booster is relying on engine vaccum. So unless your motor is having sudden vaccum surges, it cant really cause the booster to surge. I think that unhooking the booster will decrease your total stopping ability and just mask the true balance problem.

 

On the other hand, If the valve inside the proportioning valve sticks toward the front or rear system, you now have fully boosted line pressure applying to only one end of the truck. This can feel very weird.

 

You have made sure that the calipers are releasing properly and that you dont have any fluid leaks? Have you adjusted on the pedal push rod under the dash?

[Z-train]

That was un-hook the booster to see what changes.

[datzenmike]

Your 3/4" master cylinder is way too small for the much larger front calipers. A larger one would decrease pedal travel and increase effort. This would bring it back into balance. I doubt the HB one (13/16") will bolt to your booster but the 7/8" from a 720 might. Just the master cylinder that is.

Edited June 10, 2009 by datzenmike

[yello620]

Master cylinder that bolt to the 620 booster that i know of.

 

early drum 620 72-77 3/4" bore

late disc 620 78/79 13/16" bore

280 dont know the years 7/8" bore

280zx first two years 15/16" bore designed for 4 wheel disc.

[mklotz70]

Isn't the '79 280zx a disc/drum m/c? I know we've gone over this before and I was off a year last time too :)

[slodat]

all 280ZX had disc/disc. All 240z/260z/280z disc/drum.

[datzenmike]

all 280ZX had disc/disc. All 240z/260z/280z disc/drum.

 

I've heard that most of the 510s were front disc too. :D

[sssr20det510]

Check your rear brake adjustment if they are out of adjustment can cause this issue too

[BarryA]

master cylinder (had planned to step up to a 7/8 or 15/16 diameter unit anyhow - sounds like it's time to get going on that upgrade).

[Z-train]

Barry-the M/C on there now is the "correct" one for the truck?