dieseldoc Posted February 7, 2011 Share Posted February 7, 2011 Have a customer bringing me a truck some time this week he is tired of fighting with he said. The truck in question is a 94' F-250 4x4 with a 7.3 dit and a 5 spd. The pedal is ok but a little spongey, when you push it hard(panic stop or quick stop) than it goes to the "floor". If you apply the pedal gently it will stop as normal. He has cleaned, inspected, and adjusted the rear brakes and inspected the calipers and pads, all ok he says. When he first talked to me about it I told him it sounds as though there is a leak somewhere or the system has air in it. He told me he cant find a leak anywhere and has bled the system twice. He put a new master cylinder and booster on. If you unplug the vacuum line from the booster the pedal will feel normal and you can stand on it it wont go budge. As soon as you plug in the booster the pedal feels like crap. I of course am going to start with the basics myself but wondering if anybody has ever had a proportioning valve give trouble like this? I dont believe this vehicle has RABS, but not sure I have not looked at it myself yet. Your thoughts are appreciated sirs Quote Link to comment Share on other sites More sharing options...
mchan68 Posted February 7, 2011 Share Posted February 7, 2011 I have replaced more than my share of RABS valves for the exact symptom you describe on many mid-90s trucks, and I'm pretty sure all of the trucks of that era were equipped with RABS at least, if not 4WABS. Quote Link to comment Share on other sites More sharing options...
Bruce Amacker Posted February 7, 2011 Share Posted February 7, 2011 It sounds like it has air in the lines to me. Don't change any parts until you do the proper diagnostics- if it has RABS (and it probably does) look for a rubber plug or aluminum cap that can be removed at one end of the big "wheel cylinder" looking thing (RABS accumulator and hold/dump valves) on the frame rail under the driver. Remove the rubber plug or metal cap and stick a piece of coathanger (or similar feeler) inside and have an assistant push on the brake pedal. If the dump valve is stuck open, when the guy pushes on the brake pedal the piston inside will move and that is your problem. It should not move unless the truck is in an ABS stop. I've worked on so many old shitty trucks and had so many arguments over this with my techs I can tell you that most of the time it's air, not a bad RABS valve. In the process of changing the RABS valve the system is properly bled out, fixing the problem. I remember only changing one RABS valve in my life, and that was for a different issue. Other tips: 1. Put a flare plug in the MC ports or the RABS outlet and see if you can still push the pedal to the floor. 2. Disconnect the big rear feeder line that feeds the rear axle and hand bleed it at the rear "tee", 90% of the time this fixes the trouble. IIRC, this sucker is 1/4" or something big and bubbles tend to hang out in it. 2a. If someone has already changed the brake lines, make sure they are run like OE and don't have big fucking loops in them. 3. Remember that if the shoes are new or not seating into the drum properly, or the pads are worn crooked it will also cause this problem. 4. I hate doing this, but you can also clamp off the flex hoses and see if you can push the pedal to the floor to isolate your problem. 5. Check your vacuum and make sure it meets specs. 6. Remember the RABS assy also has a bleeder on it. Good Luck! Quote Link to comment Share on other sites More sharing options...
deezul Posted February 7, 2011 Share Posted February 7, 2011 ^ Awesome tips and diagnostics Bruce! This is going in the memory bank! Quote Link to comment Share on other sites More sharing options...
dieseldoc Posted February 8, 2011 Author Share Posted February 8, 2011 Thanks Bruce that is awesom tips. My plan was to pull the wheels and inspect everything myself after a little test drive to get a feel myself for what it is doing. Than I am going to bleed it myself and see if it still persists. I have a system that works pretty well for me and only takes one person. But will definitely take that line off and bleed there too. I was reading the workshop manual last night on the braking system to refresh me on what all is in the system and recomended bleeding and diagnostic procedures. Just wanted some input from you guys that have a few more of these through you doors. I dont typically get brake issues like this with these years of trucks. Thanks for the help guys, will let you know what I find when he brings it to me. Quote Link to comment Share on other sites More sharing options...
hitechmech Posted February 8, 2011 Share Posted February 8, 2011 What are the rear drums like? I had one a few years back that had new cheapest around aftermarket drums and when you applied brakes the drums flexed. Under light brake application it was fine. Changed drums and concern was corrected. Quote Link to comment Share on other sites More sharing options...
dieseldoc Posted February 9, 2011 Author Share Posted February 9, 2011 Ford replacement part about 15,000 miles ago. Had never thought would cause that type of issue. Good to know and keep in the back of a guys mind for future refference. Quote Link to comment Share on other sites More sharing options...
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