The 1947 - Present Chevrolet & GMC Truck Message Board Network - View Single Post - 47-55.1 Pressure bleeding

dsraven · 10-17-2021, 11:31 PM

I dunno if this makes sense but if you are not getting any feedback or resistance in the brake pedal until you are near the floor then it seems that the rear circuit is using a lot of travel to move fluid but not getting to the place where it builds pressure yet until late in the pedal stroke if at all, up to the point where the first piston in the master bottoms out against the second piston. like would happen if you blew a brake hose or some other failure in the rear circuit. most of the pedal travel gets taken up by that and then the second piston gets moving and you have front brakes pretty well at the end of your pedal travel. of course it seems like you have no brake action for the amount of pedal force applied because possibly the rear brakes are not doing much if anything.
maybe the next step is a flow and visual pressure test on the circuits to see what is happening. with the bleeder installed crack a line ahead of each rubber hose then do the same after each rubber hose to ensure the same flow is present with no reductions indicating a restriction and you can visually see if you think that fluid was under the 12 psi the bleeder is putting out. same for the res valves etc. work back to front eliminating possible problems as you go. even an overtightened brake fitting against a soft fitting seat can cause a deformed fitting flare or seat which becomes a restriction especially with the new style soft lines. I have seen new brake hoses that were factory overcrimped and caused a necked down inside diameter or even a new hose that was plugged in the rubber part
assuming
the system components are spec'd to work with each other as far as required output etc
the system is fully bled of air
the components are adjusted properly
the prop valve is operating properly allowing flow to both circuits
the metering valve is centered and operating properly to both circuits
the residual valves allow proper flow
no kinks or tight bends that would restrict fluid flow
no obstructed brake hoses
seems like next step is a flow and pressure test at each prop valve outlet then each bleed screw outlet
just trying to think logically about how the system should work and how best to diagnose. either start at the pump end (master cyl) checking output and flow and working downstream from there or starting at the outer ends and working back to the pump. there has to be a reason.
good on you for sticking to it. some would have given up.

10-17-2021, 11:31 PM	#18
dsraven Registered User Join Date: Nov 2010 Location: calgary alberta Posts: 7,828	Re: Pressure bleeding I dunno if this makes sense but if you are not getting any feedback or resistance in the brake pedal until you are near the floor then it seems that the rear circuit is using a lot of travel to move fluid but not getting to the place where it builds pressure yet until late in the pedal stroke if at all, up to the point where the first piston in the master bottoms out against the second piston. like would happen if you blew a brake hose or some other failure in the rear circuit. most of the pedal travel gets taken up by that and then the second piston gets moving and you have front brakes pretty well at the end of your pedal travel. of course it seems like you have no brake action for the amount of pedal force applied because possibly the rear brakes are not doing much if anything. maybe the next step is a flow and visual pressure test on the circuits to see what is happening. with the bleeder installed crack a line ahead of each rubber hose then do the same after each rubber hose to ensure the same flow is present with no reductions indicating a restriction and you can visually see if you think that fluid was under the 12 psi the bleeder is putting out. same for the res valves etc. work back to front eliminating possible problems as you go. even an overtightened brake fitting against a soft fitting seat can cause a deformed fitting flare or seat which becomes a restriction especially with the new style soft lines. I have seen new brake hoses that were factory overcrimped and caused a necked down inside diameter or even a new hose that was plugged in the rubber part assuming the system components are spec'd to work with each other as far as required output etc the system is fully bled of air the components are adjusted properly the prop valve is operating properly allowing flow to both circuits the metering valve is centered and operating properly to both circuits the residual valves allow proper flow no kinks or tight bends that would restrict fluid flow no obstructed brake hoses seems like next step is a flow and pressure test at each prop valve outlet then each bleed screw outlet just trying to think logically about how the system should work and how best to diagnose. either start at the pump end (master cyl) checking output and flow and working downstream from there or starting at the outer ends and working back to the pump. there has to be a reason. good on you for sticking to it. some would have given up.