Brake Pedal Presure

[ktlx1]

All, I have a 70 1/2 ton short bed, did a front disc conversion. The master Cylinder, prop valve, front calipers, rear brake cylinders are all new, so are the lines. I have bleed the daylights out of the truck, getting fluid to all wheels, cannot get the pedal to firm up. Any ideas, I have also checked all the lines, no leaks.

[Blue_71]

have you checked the booster? or does it have power brakes? the booster might be bad if it does

[trukman1]

Check to see if you have the correct piston in your master cylinder. Some have deep (1 1/4") piston reliefs and some have only a shallow relief in end of piston. If you use the deep piston master and need the shallow, you won't get brakes to bleed (or work) correctly. Hope this helps. Trukman1

[Stout72]

I sort of had the same problem with mine when I replaced darn near everything in the brake hydraulic system except the plumbing.

Your system may be loaded with air. Did you bench bleed your new master cylinder? VERY important.

If it's an air problem, try gravity bleeding your system. Top up your master cylinder reservoirs. Make sure you have plenty of brake fluid available. Then get 4 of those small plastic bottled water bottles and 4 pieces of clear vinyl tubing that will fit snug over your wheel cylinder/disc bleeder screws, about 12" long. Drill 2 holes in each bottle cap, one that the tubing can be crammed thru, the other for a vent. Cram a tube in each one, then press the bitter end of the tubing over the bleeders. Let the bottles hang and duct tape them to the backing plates or tire sidewalls so the weight of the brake fluid that will accumulate wont pull the tubes off the bleeders. Crack open the bleeders and let 'em drip. Then, starting at the master cylinder, crack each fitting open to make sure you're getting flow. Work your way down the system, thru the prop valve, then go around and check each bleeder. You'll probably see air bubbles thru the clear tubing. Just let the whole system gravity bleed like this for a while. Check your macter cylinder frequently and top up as needed to asure you don't run the reservoirs dry. Keep checking the bleeders. When you observe a bleeder emitting no air bubbles for several seconds, close that one off. The fronts will clear first, since they're the shortest distance from the master. The driver's rear will probably finish up last. When you got all your bleeders closed, top up the master cylinder, cap it and try your pedal. If the problem was air, you should have a nice, firm pedal now. Worked for me.

[ktlx1]

All, thanks for the tips, I did not bench bleed the MC. I will gravity feed the system. They are all new lines as well, I will keep you posted.

Thanks

[Stout72]

Do bench bleed your master first. Gravity won't get all of the air out of the master. It has to be forced thru. Best done in a bench vise, but can be done on the truck w/ 2 people. If you do it on the truck, make sure your helper is careful to depress the pedal in very short strokes. The master cylinder piston is supposed to be stroked in only about 3/4" to 1", waiting about 20 seconds between strokes. You can't really see this with it bolted on to the booster. If a master cylinder is over-stroked during the bench bleeding process, it can be damaged. Since you were working the pedal on an unbled master, this may be the case. Bench bleed your master first. If you get the air out of it, go with the gravity bleed. Otherwise you're looking at replacing the master cylinder again. Sorry for the potentially bad news. Hope it's not the case for you. Good luck.

[Southpa]

Yep, properly bleeding a master cyl. can take a lot of stroking. You might be pumping away and getting one single tiny bubble out at a time. Keep on stroking until ALL the air is out.

[72chevyLWB]

ummmmm, what were you talking about

[Brad]

I was watching a golf instructional show the other day and I couldn't believe my ears when the guy said "just visualize the hole and stroke it." I just about died laughing! :p

[jhow66]

This may not be the case with yours,but on most stock setups you have to keep the button on the end of your prop. valve depressed to bleed the brakes right. (just did a 71 this way for a buddy). Hope this helps.
www.geocities.com/jhow66