I have a 1999 4 wheel drive GMC Suburban with a soft brake pedal. It has ABS and I have replaced all components except the abs brake pressure modulation valve which also includes the proportioning valve. My question is this, Has any one other than me ever experienced this and will replacing this valve eliminate the problem ? I don't want to spend the almost $850.00 only to find out it did not take care of the situation. Yes I have replaced the booster the master both front calipers and hoses the rear wheel cylinders and brake pads and shoes including all the hardware. So if any one has any suggestions I would be very appreciative if you could help me out. Thanks to all ~!~
before you go any farther... stop..you did not say which truck you have.. 1/2, 3/4, 1 ton.. please examine the surface finish on the drums and rotors...please check the rear wheel/axle bearings for vertical play.. pull the wheel.. pull the drum.. grab the axle .. look behind it at the end of the housing.. lift the axle shaft.. do you see any movement up and down inside the end of the housing.. this is taking that you have a semi floating axle.. not a full floating axle that you have to remove the ring of bolts to get the shaft out. if you have semi floating axles.. the bearing rollers ride directly on the shaft in some... when the bearings wear or the axle surface wears.. it allows the axle to become off center with the housing.. since the shoes are mounted to the housing.. and need to be centered.. if you have axle vertical play.. the brake pedal will be SOFT and nothing will fix it without replacing the axle bearings and probably the axles.. depends upon the wear..while you have the drum off.. check for a ridge around the opening of the drum on the wear surface... that ridge did not just show up.. the smooth area that is next to the ridge was warn down.. warn drum breaking surfaces wear in a bell shape.. with the open side of the drum being larger.. this causes the brake shoes to contact the outer surface first. then try to flex to make contact all the way across.. this leads to SOFT PEDALS... machine if its within spec or replace them... you will probably want to replace or have them machined anyway.. you can take bright flashlight.. climb under the truck .. look at the backing plates.. pull the rubber plug out from the adjustment slot.. look in and identify the star wheel... use a screw driver to shove it toward the back... not turning it.. (________|__) then shove it forward..(______|___) very little movement would be good... if you find more than 1/8 of an inch.. you may have trouble someplace..now.. lets go to the front of the truck...before you pull the front wheels.. but with them jacked up.. stick a pry bar under them.. with the tires about an inch up... see if there is any visible play that would be in the front wheel bearings.. take the front tires off.. check the rotors for a wear ridge... if so... time to get them turned or replace them... again.. taper across the wear surface will cause a soft pedal...this is hard to explain.. but say... its a 12 inch diameter rotor.. and the inner diameter of the wear surface is 8 inches... that gives you 2 inches of wear area on both sides... 12 inches x pi is 37.7"8 inches x pi is 25.1"so the outer diameter moves almost 38 inches. the inner diameter moves about 25 inches.. the outer area wears faster.. this creates the taper...when you put new pads.. that are parallel .. and place them in calipers that are parallel .. then put them against a rotor that has a significant amount of taper... the inner edge of the pad touches first.. then something has to flex to bring it all into contact.. this will also give you a soft brake pedal... ~~~~~~~~~~~~~~~~~lastly... i have also had issues... with the caliper slides... where people have improperly greased them... using silicone dielectric grease is a NO NO... silicone dielectric grease has dissolved SAND in it.. when heated by the brake system.. the sand comes out of its liquid form.. becomes sand again... and prevents the caliper slides from moving freely... i have found some that would not budge with the 30 ton shop press.. yep i tried.. why.. when the brake pedal is depressed.. the fluid pushes out on the caliper pistons.. this pushes the inner pad against the inside of the rotor.. instead of flexing the rotor.. the caliper slides and the outer pad comes in to equal the pressure exerted by the inner pad.. if the caliper sides are not free.. you will bend the rotor to meet the outer pad.. ever tried to bend cast iron.. it will slightly... you can also find that the caliper stays put. and the outer pad wears as the calipers sides are too tight to allow it to release..the front wheel bearings could also be slightly worn .. allowing the rotor to fit sideways through the brake pads in the caliper.. the closing force has to straighten the rotor and the wheel... again a soft pedal...please copy this and print it.. this is a lot to absorb..
WOW if you have spent that much do print Wayne's information.... That is privelaged information....LOL... That's info you only get from someone thats been there and done that.Now do you have a Vacuum assisted ABS System, or a Hydraboost assisted ABS System.If your not sure most 1500 Trucks Chevy or GMC will have a Vacuum assist brake system. Almost same the Chevy/GMC, and all 2500HD,3500Hd and diesels will have a Hydraboost braking system. The vacuum system will have a big Vacuum canister holding unit on the back nearest the firewall just in front of the master cylinder.The Hydraboost system will have a Master cylinder nearer the firewall, with a Black Hydraulic pressure system in front of it. In short it is a hydraulic assited unit that has a pump that is a power steering pump and also supplies assisted hydraulic pressure to the braking system. If Wayne's diagnosis fixes your problem great. You will want to print this information also. If Wayne's diagnosis doesn't work let us know I will finish.Bob
It's possible you have air in the system...Bob