Here's what I have. new Dual 7" booster, master cylinder 1-1/8, brass proportional valve, 2 lb. residual valve front and one towards the back. I checked the distance between booster and MC pedal plug, all parts under floorboard. Installed disc brake kit on 9" ford rear. Bled the system 10 times or more. Once I start the truck the brake pedal goes to the floor. Have 17' vacuum. There is caliper movement, but at the end of pedal travel and just enough to hold truck from moving in gear. Kit was from MBM Company and installed with no problems. Is there a need for an adjustable proportional valve also? Thanks
I struggled with the 4 wheel disk set up I used under my DeSoto wagon. I started, like you, with a 7" dual booster and 1 1/8" bore MC. Although I had a hard pedal, the rear brakes didn't work at all. I ended up replacing EVERYTHING, went to a larger booster (I think it's 8", no room on the firewall for a single, larger one), 1" bore MC, ditched the (two that I tried) proportioning valves meant for 4 wheel disk brakes and used an adjustable one. Initially, after buying a pressure gauge, I found I had just under 500 PSI to the rear, (Explorer 8.8), which need 1,000 PSI. Given that your problem sounds similar to what I struggled with, first, I'd get a pressure gauge so you know where you are, and replace the MC with a 7/8" or 1". Good luck.
Pray tell us, what say you? Sounds like a vacuum leak or he simply isn't getting the required 1000 lbs plus. So many similar problems on here, invariably someone will suggest a separate forum. I know! call professor Hopkins at Midas, no wait it's too long distance
Many time when the pedal goes to the floor when the engine is started, is because the link between the master and the booster is adjusted too long, activating the booster when the engine produces vacuum.
I'm not sure when hairs became an official form of measurement, but the blond ones I've seen were not a thick as some of the blacks and browns. I think you have air in the system. But since I have to ask anyhow, was it bench bled before installing? Was it further bled with the car off or on while running? You bled it 10 times and how exactly was that accomplished? Like I mentioned, I think you haven't bled it properly, but what do I know...? Well, I know I have a Kevin and that's what he'd say. Air in the system. Heck, I'm to assume you have bled it properly, when the possiblility is, I'm over estimating that what you have done, and that it was done right. With all due respect, at the end of the push stroke when bench bleeding a master cylinder, that's when you really need to push, and when most don't or fail to do so fully and slowly. They think they are humping a pig in the dark and short fast strokes are going to do it, it doesn't. So...you're not building pressure and you don't have a leak, what could it be?
It Was bench bled, I bled the brakes a few different ways. with a buddy pumping and holding, with me pumping and holding the peddle down with a stick. , With me pumping once hold with stick and tapping caliper with hammer. I have a pedal somewhat without starting the truck, as I start the pedal goes away, in gear (automatic) just barely holds the truck in place. Was tempted to put drum brakes back on. It's probably something real stupid looking me in the face. It's going to a local mechanic on Tuesday. I also used the vacuum type tool , which isn't much good. Thanks
The pedal cannot move on its own, unless there is not enough free play in either or both the pushrod between the pedal and booster, or the booster and the master cylinder. It is acting like you are pushing the pedal. You need to find that absence-of-clearance and fix that. Also, the pedal cannot go to the floor unless you either have a leak, or you have air in the system, or you have a pair, or two pair (if your are running non-parking brake calipers, aka: front calipers) GM "low drag" calipers. If you had a leak big enough for the pedal to go to the floor, you would have a very large, and very visible puddle. If you are running GM parking brake calipers, they need to be actuated manually to take up the gap between the pads and the rotors. New, or remanufactured, they are put back to "maximum out" position. It's not all that unlike setting shoe drag on a drum setup. As has been mentioned, bench bleeding a master cylinder involved moving the entire stroke that the master cylinder is capable of, as many times as it takes for no more bubbles to come out for about 6 more pushes. This really should not be done with the master cylinder in the vehicle. I only recommend doing it that way when you are in the auto parts store parking lot, on a road trip. This can, and should work just fine. Drum brakes were just fine for the days when that was the standard of the day. That time has passed. Anyone that tells you that a properly maintained drum brake system works just as well as a disc system is delusional, does not understand the Laws-of-Physics, does not understand how much lawyers cost, are unaware that insurance companies do investigate accidents that involve bodily harm, and/or has a death wish for themselves, that they are trying to impart to you, and anyone that rides with you, or drives near you. Don't join that cult. Take the master cylinder out. Put it on the bench, in a vise, and re-bleed it there. If that does not do it, it is entirely possible that the master cylinder is bad. If the seals were damaged when it was re/built, or they dried out over time, brake fluid will just pass by them and go back to the reservoirs, without actuating the calipers. I have had more than a dozen master cylinders that have been dead, new-in-box. I have had too many examples of GM low-drag calipers randomly mixed in with the like-appearing GM ones. I have had probably a dozen GM parking brake rear calipers, both pre-metric, and Metric versions, that had their parking brake mechanism hopelessly "pre-jammed" new-in-box. At 65 mph, keeping one second of following distance means the vehicle is traveling 100 feet behind the vehicle in front. It takes an automobile a minimum of 150 feet to stop. The average vehicle in the US fleet is 12-years-old, and is fully capable of stopping 60-feet shorter from 65 mph than the last of the all-all-drum-brake vehicle. 65 mph is 95.33 feet-per-second. That means that even if you brake at the same instant, with zero delay, you will very likely hit the vehicle in front of you!
You said you checked the master to the booster rod gap, but again I ask that you check the pedal to the master rod gap. If no gap the pedal is pushing the booster, which pushes the master, which applies the brakes.
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