Brake Problems- 72 3/4 ton 4x4 - The 1947 - Present Chevrolet & GMC Truck Message Board Network

71 super · 07-16-2009, 08:47 PM

My little rother has a 72 3/4 ton 4x4 he uses to pull his boat with. Loads boat up two weeks ago and starts out, leaving a fluid line out the driveway. No visable leaks. No cracked lines no expanded hoses etc. but rear reservior is empty (controls the front brakes) We replaced m/c today since it is realativly cheap and easy. We got some brake back but the dash light is still on and low pedal. Cylinder was bench bled and brakes were bled after installed on the truck, but brakes are weak at best. Any ideas I am leaning toward prop valve issue - reset or ??? or diaphram problem with the booster. Any ideas or suggestions are appreciated. How do you reset a prop valve to get light to turn off also would help. Thanks Harold

**special-K** · 07-16-2009, 10:28 PM

When it left a trail of fluid,where did it come from on the truck?Wasn`t it wet anywhere?Which side of the truck was the trail left on?

71 super · 07-17-2009, 09:43 AM

Tim I have crawled all over the underside of the truck and could not find a single location that had brake fluid on it. I did not see this when it occurred he called after he got it back to the garage the next day. The only location that even looked like it had fluid on it was the front rubber bumper on the driver's side. I can find no cracked lines, bulging rubbers etc. at all. There was no evidence of any leaking on wheels around calipers or at wheel cylinders. After replacing master it sounds as if only the rear brakes are working. Do I need to depress pin in prop valve and rebleed? I looked in service manual and It indicated that the pin should be depressed each time the brakes were bleed- I have never in all of the 1/2 ton trucks I have had and done brakes on needed to do this before... any ideas are appreciated. Harold

tucnnuts · 07-18-2009, 10:37 PM

not sure but i think the prop valve switch only works if the pressure diff between the front and rear is off..has it stoped using fluid with the new master?

71 super · 07-19-2009, 12:40 AM

We bled on the new master for two days and stil no pedel, thought what the hay and pulled the new m/c and guess what, fluid coming out of the rear of the new cylinder. We ordered a new one yesterday and hope to install it monday so we shall see if the brakes return and the prop valve resets.

07-16-2009, 08:47 PM	#1
71 super Registered User Join Date: Sep 2005 Location: Canton North carolina Posts: 3,080	Brake Problems- 72 3/4 ton 4x4 My little rother has a 72 3/4 ton 4x4 he uses to pull his boat with. Loads boat up two weeks ago and starts out, leaving a fluid line out the driveway. No visable leaks. No cracked lines no expanded hoses etc. but rear reservior is empty (controls the front brakes) We replaced m/c today since it is realativly cheap and easy. We got some brake back but the dash light is still on and low pedal. Cylinder was bench bled and brakes were bled after installed on the truck, but brakes are weak at best. Any ideas I am leaning toward prop valve issue - reset or ??? or diaphram problem with the booster. Any ideas or suggestions are appreciated. How do you reset a prop valve to get light to turn off also would help. Thanks Harold __________________ 67 blue step side- bb 68 gold and white swb cst anniversary 68 custom swb hot rod Ls swap 69 swb cst 6 cylinder/ overdrive 69 black/black cst bb swb 70 orange and white cst Canada built truck swb bb 71 black and white super swb bb 71 red and white super swb 72 green and white super swb 4x4 62 nova wagon 400 series Ls swapped 68 Camaro 327/4 speed- 1 owner, original paint 66 Volkswagen bettle 69 auto stick Volkswagen Beetle Last edited by 71 super; 07-16-2009 at 08:48 PM.

07-16-2009, 10:28 PM	#2
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,851	Re: Brake Problems- 72 3/4 ton 4x4 When it left a trail of fluid,where did it come from on the truck?Wasn`t it wet anywhere?Which side of the truck was the trail left on? __________________ "BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~

07-17-2009, 09:43 AM	#3
71 super Registered User Join Date: Sep 2005 Location: Canton North carolina Posts: 3,080	Re: Brake Problems- 72 3/4 ton 4x4 Tim I have crawled all over the underside of the truck and could not find a single location that had brake fluid on it. I did not see this when it occurred he called after he got it back to the garage the next day. The only location that even looked like it had fluid on it was the front rubber bumper on the driver's side. I can find no cracked lines, bulging rubbers etc. at all. There was no evidence of any leaking on wheels around calipers or at wheel cylinders. After replacing master it sounds as if only the rear brakes are working. Do I need to depress pin in prop valve and rebleed? I looked in service manual and It indicated that the pin should be depressed each time the brakes were bleed- I have never in all of the 1/2 ton trucks I have had and done brakes on needed to do this before... any ideas are appreciated. Harold __________________ 67 blue step side- bb 68 gold and white swb cst anniversary 68 custom swb hot rod Ls swap 69 swb cst 6 cylinder/ overdrive 69 black/black cst bb swb 70 orange and white cst Canada built truck swb bb 71 black and white super swb bb 71 red and white super swb 72 green and white super swb 4x4 62 nova wagon 400 series Ls swapped 68 Camaro 327/4 speed- 1 owner, original paint 66 Volkswagen bettle 69 auto stick Volkswagen Beetle Last edited by 71 super; 07-17-2009 at 09:44 AM.

07-18-2009, 10:37 PM	#4
tucnnuts if they can make it so can I! Join Date: Feb 2008 Location: birmingham, alabama Posts: 97	Re: Brake Problems- 72 3/4 ton 4x4 not sure but i think the prop valve switch only works if the pressure diff between the front and rear is off..has it stoped using fluid with the new master?

07-19-2009, 12:40 AM	#5
71 super Registered User Join Date: Sep 2005 Location: Canton North carolina Posts: 3,080	Re: Brake Problems- 72 3/4 ton 4x4 We bled on the new master for two days and stil no pedel, thought what the hay and pulled the new m/c and guess what, fluid coming out of the rear of the new cylinder. We ordered a new one yesterday and hope to install it monday so we shall see if the brakes return and the prop valve resets. __________________ 67 blue step side- bb 68 gold and white swb cst anniversary 68 custom swb hot rod Ls swap 69 swb cst 6 cylinder/ overdrive 69 black/black cst bb swb 70 orange and white cst Canada built truck swb bb 71 black and white super swb bb 71 red and white super swb 72 green and white super swb 4x4 62 nova wagon 400 series Ls swapped 68 Camaro 327/4 speed- 1 owner, original paint 66 Volkswagen bettle 69 auto stick Volkswagen Beetle



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