For those with vacuum and 12" front discs...

stickybones

What?
Joined
Dec 27, 2001
What size master cylinder are you using? I have the LS1 front brake conversion, "stock" rear drums with the two big shoes and S10 size brake cylinder and new brass proportioning valve. I'm still not satisfied with the braking power of the car. This has only come to light fairly recently as I've driven alot of miles in the other two cars in my sig. Their braking capability puts my GN to shame. My GN has a decent pedal (i.e. not too hard and not spongy), but the brakes just plain suck. I often wonder if I could lock them up on ice!:rolleyes:

Since nothing appears deficient in my set up per se, I'm wondering if I should step up a size on the master cylinder in order to displace more fluid, but with a resultant loss in downstream line pressure. I know that sounds counter-productive with my described problem, but I'm really at a loss to figure out why my brakes seem so inadequate. I no longer remember what the brakes were like when I had a full stock setup; powermaster and all, but I have to imagine they were better than this.

Thanks
Mike
 
Hi Mike, :) I'm waiting for hubs right now to do the LS1 conversion hopefully next week. What master cylinder are you using? The standard 87 NA G-body aluminum master cylinder? I haven't heard anything but positive things about the LS-1 conversion. I can't imagine why you're having a problem. I know you're quite the fabricator and mechanic, so we can rule you out as the problem. :biggrin: I'm sure you'll track down the problem. ;)
 
Hey John

Stop being sarcastic;)

I have the 24mm/36mm dual bore master. I think this the stock size for the G-body vacuum brakes.

Maybe since I'm an old-timer (had a GN now for 20yrs) , I get frustrated when I can't figure out what the heck is wrong.
 
Hey John

Stop being sarcastic;)

I have the 24mm/36mm dual bore master. I think this the stock size for the G-body vacuum brakes.

Maybe since I'm an old-timer (had a GN now for 20yrs) , I get frustrated when I can't figure out what the heck is wrong.

I don't know what to suggest to you other than to double check everything and track down the problem. :frown: Maybe someone can chime in with more specific suggestions. ;)
 
Just guessing but how about a late 70's to early 80's B-Body. They came with 12" front disk and drum rears.
Conrad
 
are calipers new....master cyl. should match calipers,how much vacuum at booster and where are you plumbed at motor for vacuum....when my rears where overheating and causeing fade i could never lock the front wheels...it is possible all 4 discs drums are glazed and you will never be happy with performance you should test oil pressure at calipers and wheels cyls.
 
are calipers new

No. When I did this conversion a few years ago, I bought used LS1 calipers and rebuilt them before I installed them.

....master cyl. should match calipers
That's my original question.

,how much vacuum at booster and where are you plumbed at motor for vacuum
~15in-hg with one of those fancy vacuum blocks on the top of the throttle body (i.e. dedicated line for the brake booster)

....when my rears where overheating and causeing fade i could never lock the front wheels...it is possible all 4 discs drums are glazed and you will never be happy with performance

I just got the front discs turned so that shouldn't be a problem. I turned them because I had this annoying vibration thru the brake pedal if I apply the brakes hard. I thought maybe the discs were warped. The vibration hasn't gone completely away so it must be something else; possibly symptomatic of my overall problem. I didn't mess with the rears other than making sure the drums just slid over the shoes.

you should test oil pressure at calipers and wheels cyls.
That would be nice, but I see a real mess on my hands doing so in addition to needing some kind of special fittings to get gauges in there. Furthermore, I wouldn't even know what the line pressure at the wheels should be.

Thanks for the food for thought.
Mike
 
Hey Mike .. how about your vacuum to the booster? Maybe it's the booster. Defective booster or inadequate vacuum would result in harder, ineffective, pedal. I think the MC is big enough.
 
the pulsating is probably in the back ,you have to much modifications 4 long shoes plus big wheel cylinders ,the drums are wraped the and the spring kit has lost all tension....INSTALL 2 MADE IN THE USA DRUMS NO CHINA NEW SPRINGS AND REGULIAR SOFT SHOES ,,,CHECK EMERGENCY BRAKE CABLES MUST BE FREE AND DONT OVER HEAT FOR COUPLE OF DAYS
 
STICKY,remove the rear drums and look for stress lines (cracking) on surface of linings and even on drum where lining applys again major heat which will cause fade and car will only brake useing front or 60% of brakes SOORRY FOR BABBBALING ON
 
I AM BORED...and if you take of the rear drums and all looks new ,it is possible that when you bleed the system (by foot) you caused the propor. valve to jump and now you have no oil pressure to back to back wheels????
 
I was thinking of getting new drums for the rears thinking the pulsating was coming from there after cutting the front discs didn't cure it. Not sure how to find "made in USA" drums though.
 
just buy good quality not low price and no 2 long shoes per side and springs if toasted..,the big wheel cyls are ok as this will balance out with the bigger brakes in front...I went through about the same problem as you ,i just dont know if your system is properly blead or if prop. valve is not malfunctioning .. my car will pitch me trough the windshild ..did you follow proper bleeding procedures
 
I was thinking of getting new drums for the rears thinking the pulsating was coming from there after cutting the front discs didn't cure it. Not sure how to find "made in USA" drums though.


Mike, did you miss my post? I'm pretty sure your booster is the problem.
 
I got my booster from you years ago.:eek:

Do boosters go partially bad? My pedal isn't hard.

Does a bad booster cause the pedal pulsation/vibration I feel when pushing hard?

Well, it just seems to me you don't have sufficient power assist. Perhaps you're running a hi-performance cam? I just can't see why the car won't stop. One of your components must be bad. I have noticed that some boosters provide better stopping than others. I had an NA Regal some years back and it would put you through the windshield. You say your pedal isn't hard. Now I know what you're saying, but don't you have to press hard for it to stop? Obviously you have some power assist, but the power assist is supposed to do the muscle work. I am speculating but when all components appear to be up to snuff, but the car doesn't stop, everything gets called into question. I don't know about the pulsing. I'd think that's something out of round but I'm no brake engineer. :D
 
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