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Brake pressure prob

812 views 6 replies 2 participants last post by  Abe's 51 
#1 ·
51 gmc on s10 frame. Stock s10 frount disc. S10 blazer disc rear. Firewall mount 87 corvette booster m/c combo 1&1/8 bore.900 psi at rear bleeders.1100 psi out of both m/c ports.1000 psi at frount hard lines but ,only 500-700 psi at frount bleeders. All new parts,bleed system many times. Good pedal,truck stops but won't lock up in panic stop. 3/16 line to frount,1/4 to rear. Adjustable pro.valve wide open to rear. I'm lost ,any ideas?
 
#2 ·
depending on the year of the blazer rear axle.. if it uses snap ring retained wheel cylinders .. one year and one model only used larger diameter wheel cylinders that will increase the rear brake bias...

i think it was a 1986 S10 blazer 2.8.. instead of 3/4" it used 13/16 wheel cylinder bores.. this helps a lot of circle track guys who went to impala front spindles on G body cars get the brake bias closer to what it should be..

please be sure that you use something like a C clamp and a socket to install the snap rings.. the fingers have to go into the latches or they will come loose.. perhaps causing loss of fluid/pressure at the worst possible time.

what other devices are in your system... brake combo valve?? look closely.. both sides have some valving in them. hmm..

and do you have nice fat sticky tires??
 

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#4 ·
lines direct from the master cylinder to the calipers.. no prop valve or antilock control valve hooked in??

do the rear calipers have a parking brake lever on the caliper??? that if the rear calipers internal parking brake adjustment is low.. it will take extra fluid volume to fill the rears and throw off the system in the front..

why do i say that.. i am wondering how you measured pressure at the master..

i personally would like to see much higher pressures from the master but i don't know how much push and how much manifold vacuum you have .. to assist in building brake booster vacuum.. need to really have at least 14 inches of vacuum in there. gm and a few other cars in the mid 80s thru early 90s used power brake vacuum booster pumps.. usually hidden down in the fender.. they had two types.. one with a vacuum switch built in and the other that used a vacuum switch on the booster face.. these would not create enough vacuum to assist the brake application unless they were hooked up with manifold vacuum on one side and the brake booster on the other. they could boost the vacuum readings 5 or 6 inches.. but that was it.

so.. how much manifold vacuum do you have?? available for your booster?

what else is in line from the master to the front calipers.. i cannot see your car brake system routing thru the internet. my connections a little fuzzy.

is there one of the black fuel filter devices in the power brake booster hose.. that is a carbon vapor trap and they do clog reducing the air flow out of the booster slowing down the build up of vacuum to assist.. dorman/help sells replacements if its clogged.

why am i worried about the rear caliper adjustment.. inside the master cylinder the rear piston pushes the forward piston with brake fluid .. not metal to metal contacts.. too much fluid used on one side or the other will result in a low pedal that does not stop the car..

and i think that corvette master may be one of the quick take up versions.. but i am not the brake specialist.. i only dabble and mash them when i need to stop..
 
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