Brake Pedal Presure - The 1947 - Present Chevrolet & GMC Truck Message Board Network

ktlx1 · 09-14-2002, 04:25 PM

All, I have a 70 1/2 ton short bed, did a front disc conversion. The master Cylinder, prop valve, front calipers, rear brake cylinders are all new, so are the lines. I have bleed the daylights out of the truck, getting fluid to all wheels, cannot get the pedal to firm up. Any ideas, I have also checked all the lines, no leaks.

Blue_71 · 09-14-2002, 04:38 PM

have you checked the booster? or does it have power brakes? the booster might be bad if it does

trukman1 · 09-14-2002, 05:18 PM

Check to see if you have the correct piston in your master cylinder. Some have deep (1 1/4") piston reliefs and some have only a shallow relief in end of piston. If you use the deep piston master and need the shallow, you won't get brakes to bleed (or work) correctly. Hope this helps. Trukman1

Stout72 · 09-15-2002, 12:32 PM

I sort of had the same problem with mine when I replaced darn near everything in the brake hydraulic system except the plumbing.

Your system may be loaded with air. Did you bench bleed your new master cylinder? VERY important.

If it's an air problem, try gravity bleeding your system. Top up your master cylinder reservoirs. Make sure you have plenty of brake fluid available. Then get 4 of those small plastic bottled water bottles and 4 pieces of clear vinyl tubing that will fit snug over your wheel cylinder/disc bleeder screws, about 12" long. Drill 2 holes in each bottle cap, one that the tubing can be crammed thru, the other for a vent. Cram a tube in each one, then press the bitter end of the tubing over the bleeders. Let the bottles hang and duct tape them to the backing plates or tire sidewalls so the weight of the brake fluid that will accumulate wont pull the tubes off the bleeders. Crack open the bleeders and let 'em drip. Then, starting at the master cylinder, crack each fitting open to make sure you're getting flow. Work your way down the system, thru the prop valve, then go around and check each bleeder. You'll probably see air bubbles thru the clear tubing. Just let the whole system gravity bleed like this for a while. Check your macter cylinder frequently and top up as needed to asure you don't run the reservoirs dry. Keep checking the bleeders. When you observe a bleeder emitting no air bubbles for several seconds, close that one off. The fronts will clear first, since they're the shortest distance from the master. The driver's rear will probably finish up last. When you got all your bleeders closed, top up the master cylinder, cap it and try your pedal. If the problem was air, you should have a nice, firm pedal now. Worked for me.

ktlx1 · 09-15-2002, 12:37 PM

All, thanks for the tips, I did not bench bleed the MC. I will gravity feed the system. They are all new lines as well, I will keep you posted.

Thanks

Stout72 · 09-15-2002, 01:12 PM

Do bench bleed your master first. Gravity won't get all of the air out of the master. It has to be forced thru. Best done in a bench vise, but can be done on the truck w/ 2 people. If you do it on the truck, make sure your helper is careful to depress the pedal in very short strokes. The master cylinder piston is supposed to be stroked in only about 3/4" to 1", waiting about 20 seconds between strokes. You can't really see this with it bolted on to the booster. If a master cylinder is over-stroked during the bench bleeding process, it can be damaged. Since you were working the pedal on an unbled master, this may be the case. Bench bleed your master first. If you get the air out of it, go with the gravity bleed. Otherwise you're looking at replacing the master cylinder again. Sorry for the potentially bad news. Hope it's not the case for you. Good luck.

Southpa · 09-15-2002, 01:15 PM

Yep, properly bleeding a master cyl. can take a lot of stroking. You might be pumping away and getting one single tiny bubble out at a time. Keep on stroking until ALL the air is out.

72chevyLWB · 09-15-2002, 03:06 PM

ummmmm, what were you talking about

Brad · 09-16-2002, 11:33 PM

I was watching a golf instructional show the other day and I couldn't believe my ears when the guy said "just visualize the hole and stroke it." I just about died laughing! :p

jhow66 · 09-17-2002, 08:58 AM

This may not be the case with yours,but on most stock setups you have to keep the button on the end of your prop. valve depressed to bleed the brakes right. (just did a 71 this way for a buddy). Hope this helps.
www.geocities.com/jhow66

09-14-2002, 04:25 PM	#1
ktlx1 Registered User Join Date: Sep 2002 Location: Clover, SC Posts: 15	Brake Pedal Presure All, I have a 70 1/2 ton short bed, did a front disc conversion. The master Cylinder, prop valve, front calipers, rear brake cylinders are all new, so are the lines. I have bleed the daylights out of the truck, getting fluid to all wheels, cannot get the pedal to firm up. Any ideas, I have also checked all the lines, no leaks.

09-15-2002, 12:32 PM	#4
Stout72 Not my good side. Join Date: Mar 2002 Location: Fairfield, California Posts: 222	I sort of had the same problem with mine when I replaced darn near everything in the brake hydraulic system except the plumbing. Your system may be loaded with air. Did you bench bleed your new master cylinder? VERY important. If it's an air problem, try gravity bleeding your system. Top up your master cylinder reservoirs. Make sure you have plenty of brake fluid available. Then get 4 of those small plastic bottled water bottles and 4 pieces of clear vinyl tubing that will fit snug over your wheel cylinder/disc bleeder screws, about 12" long. Drill 2 holes in each bottle cap, one that the tubing can be crammed thru, the other for a vent. Cram a tube in each one, then press the bitter end of the tubing over the bleeders. Let the bottles hang and duct tape them to the backing plates or tire sidewalls so the weight of the brake fluid that will accumulate wont pull the tubes off the bleeders. Crack open the bleeders and let 'em drip. Then, starting at the master cylinder, crack each fitting open to make sure you're getting flow. Work your way down the system, thru the prop valve, then go around and check each bleeder. You'll probably see air bubbles thru the clear tubing. Just let the whole system gravity bleed like this for a while. Check your macter cylinder frequently and top up as needed to asure you don't run the reservoirs dry. Keep checking the bleeders. When you observe a bleeder emitting no air bubbles for several seconds, close that one off. The fronts will clear first, since they're the shortest distance from the master. The driver's rear will probably finish up last. When you got all your bleeders closed, top up the master cylinder, cap it and try your pedal. If the problem was air, you should have a nice, firm pedal now. Worked for me. __________________ 72 Chevy K20 Custom Camper, 350/350, 4" lift, dual shock suspension front & rear. Daily driver and a work in progress. 00 Suburban LT 90 Camaro RS 79 Yamaha XS1100 Special (Ol' Reliable)

09-15-2002, 01:12 PM	#6
Stout72 Not my good side. Join Date: Mar 2002 Location: Fairfield, California Posts: 222	Do bench bleed your master first. Gravity won't get all of the air out of the master. It has to be forced thru. Best done in a bench vise, but can be done on the truck w/ 2 people. If you do it on the truck, make sure your helper is careful to depress the pedal in very short strokes. The master cylinder piston is supposed to be stroked in only about 3/4" to 1", waiting about 20 seconds between strokes. You can't really see this with it bolted on to the booster. If a master cylinder is over-stroked during the bench bleeding process, it can be damaged. Since you were working the pedal on an unbled master, this may be the case. Bench bleed your master first. If you get the air out of it, go with the gravity bleed. Otherwise you're looking at replacing the master cylinder again. Sorry for the potentially bad news. Hope it's not the case for you. Good luck. __________________ 72 Chevy K20 Custom Camper, 350/350, 4" lift, dual shock suspension front & rear. Daily driver and a work in progress. 00 Suburban LT 90 Camaro RS 79 Yamaha XS1100 Special (Ol' Reliable)

09-15-2002, 01:15 PM	#7
Southpa Registered User Join Date: May 2000 Location: Victoria, BC, Canada Posts: 2,410	Yep, properly bleeding a master cyl. can take a lot of stroking. You might be pumping away and getting one single tiny bubble out at a time. Keep on stroking until ALL the air is out. __________________ 1970 GMC 1500 Custom Original 350/TH350 Victoria, BC, Canada You can wish in one hand and crap in the other. See which one gets filled first.

09-15-2002, 03:06 PM	#8
72chevyLWB Got 20's???? Join Date: Oct 2001 Location: Aberdeen SD Posts: 1,451	ummmmm, what were you talking about __________________ 99 Silverado Ex-Cab 2wd, Bagged&WhippleCharged 02 Sierra Ex-Cab 4wd 2500 HD The 72 went down the road.

09-14-2002, 05:18 PM	#3
trukman1 Registered User Join Date: Apr 2001 Location: Beavercreek, Ohio 45431 Posts: 737	Check to see if you have the correct piston in your master cylinder. Some have deep (1 1/4") piston reliefs and some have only a shallow relief in end of piston. If you use the deep piston master and need the shallow, you won't get brakes to bleed (or work) correctly. Hope this helps. Trukman1

09-15-2002, 12:37 PM	#5
ktlx1 Registered User Join Date: Sep 2002 Location: Clover, SC Posts: 15	All, thanks for the tips, I did not bench bleed the MC. I will gravity feed the system. They are all new lines as well, I will keep you posted. Thanks

09-16-2002, 11:33 PM	#9
Brad Out of the carpool lane. Join Date: Jan 2000 Location: Clark Co, WA Posts: 5,673	I was watching a golf instructional show the other day and I couldn't believe my ears when the guy said "just visualize the hole and stroke it." I just about died laughing! :p __________________ 1968 C-10 SWB, 5.7 Vortec/700R4/3.73 posi, Torch Red 1968 Camaro, 250/Powerglide, all original (No, I'm not gonna drop a 350 in it!...Jeez!) 2000 Honda VFR in the faster yellow! 2008 Husqvarna TE-610 1967 C-10 SWB 'Six Appeal'-Gone but not forgotten...

09-17-2002, 08:58 AM	#10
jhow66 Registered User Join Date: Jun 2002 Location: Knoxville Tenn. Posts: 3,058	brakes This may not be the case with yours,but on most stock setups you have to keep the button on the end of your prop. valve depressed to bleed the brakes right. (just did a 71 this way for a buddy). Hope this helps. www.geocities.com/jhow66 __________________ 56 Chevy Bel-Air 2dr. HT (purchased new) 71 Chevy Cheyenne SWB PU (502HO) 65 GMC short bed step--work in progress and my gofer



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