Hey guys... a buddy has an '88 5.7L Iroc with the J65 brake option.

The car has been converted to a turbo Buick V6 and a TH400 trans. First time to the track....car went 134 mph through the traps....and had a very hard time stopping.

Mods to the brakes at that point: none.

Since then.... the original rear was pulled in favor of a Ford 9" that had been completely built for a F-Body..... it has drum brakes....torque arm... just like the factory. At this point...the rears locked up much too soon... and I advised he install an adjustable proportioning valve on the rear... and fine tune his bias.... which he did. Around the same time.... he bought a 1LE complete front brake setup off a turbo trans am.... spindles... calipers... and all... They have been installed.

He has been advised that he needs the 1LE proportioning valve to make the brakes function properly. The more I read.... I disagree. He has the factory disk/disk proportioning valve.... and an adjustable proportioning valve on the rears.... so he can get his brake bias perfect. I just don't see how a different proportioning valve can do anything any better than that.

He has purchased the 1LE proportioning valve.. but we haven't installed it. I read his car is a "1st design" proportioning valve... and likely has 1.0mm pitch threads. I read that the "2nd design" has 1.5mm pitch on the threads..... so I don't even think it will bolt up to the stock lines.

Since the motor is turbocharged.... I am leaning toward a hydroboost conversion. I think his car just isn't making enough vacuum with his cam... + it is turbocharged..... this lack of vacuum is probably causing low pressure at the calipers/wheel cylinders..... all of which can be rectified with a hydroboost conversion.

Any thoughts?

 

Is there a different cam in the motor? If it is basically stock. I garontee he has enough vacume. If you want to know for sure why not just check with a vac. gauge?

Then if he is still only pulling like 6-9" of vac. Buy a vac. res. can for $20 from summit. That will leave him with enough vac when it comes time to push the brakes.

I used to only pull 6" of vac, and I ran a can, and was fine for the street. I now pull about 9" but still run the can. Of course still fine.

Much cheaper than that hydroboost stuff.

 

It is a hydraulic roller grind ... running solid roller lifters.... not sure the exact specs.... but it does have a lope to it... so vacuum is likely fairly low.


I am thinking we might experiment with a dedicated vacuum pump just for the brakes... and a vacuum canister..... the pump would kick on during boost (off an output from the FAST XFI) or if you pushed the brake pedal. The canister would store enough vacuum to give you a few brake pumps anyway.... We have a few air pumps off lat model trucks that can be (and have been) modified to make them a vacuum pump. I am going to try and wire this up prior to going to the track next time.

Does this sound like a good plan? I wonder how many inches of vacuum that one of thos pumps can pull? I would assume the more vacuum it can pull the better the brakes will be.

 

0 vac at WOT (after a pass) so theres nothing in the reserve, i would go hydroboost. My car had a problem shutting down after a pass so i put bigger ls1 brakes on it front and rear now stops just fine

 

Yeah, but when you are idling before a pass you have already filled the reserve. there is a one way check valve you are supposed to install so that anytime you make vacuum it gets "saved" up in there for later use. I have been in the office right now for 1 hour, but i know that my car still have vacuum waiting in the can for me when I want to use it, even with the car off. So saying that you have none on reserve after a run, if you are running a can, is just plain wrong. Or installed wrong.

 

I agree you should be able to store vacuum and it be there after the run.......

I also think his cam may not be generating enough vacuum to get enough assist from the unit.

I also think the vacuum pump we have isn't going to pull enough vacuum to work real good... only 5-6 in.... I plan on measuring it soon.

I had visions of an intermittent use pump... that could generate 20 in of vacuum and provide considerably more assist than we are currently getting. I am not sure how realistic that goal is... or if 20 in of vacuum will make the brakes apply harder.

I really think the 12" 1LE brakes ought to work great..... stock TTA's came with those brakes... and they would go 162 mph right off the showroom floor.....

 

Your factory original combo vavle is shot. You are loosing brake pressure because the vavle is malfunctioning and causing blockage.

Put the new one in and it will solve your problem. The additional rear prop valve (wilwood valve I am assuming) is really not needed. It was a bandaid fix to the bad factory combo valve and the fronts degrading- you were having to turn the rears down also.

 

The bias was fine until the drum brakes were added..... then the rears locked up to easily. (at this point... the factory J65 brakes were on the front)Supposedly it takes less pressure to lock-up drum brakes than disk brakes. From everything I read.... the drums are "self energizing" which I think means that once the shoe makes contact.. they really want to apply more on their own..... and with disks you have to maintain or increase pressure to get them to hold.

As far as the factory combo valve.... we were going to replace it with a real 1LE valve... but the brake bias is still going to be off because it has the drum brakes. The 1LE combo valve is for disk/disk...... so IMHO... the chance the brake bias beeing correct is nil.