Can't lockup front brakes - bad master cyl? or something else?

[Dieselfume]

OK, been tinkering with this on and off all summer (cuz I've been in and out of town for work). 87 R-10 with JB3 brakes. Purchased in 09, but didn't get driven much the last 5-8 years, maybe 1-2k mi.

Since I've purchased the truck in 09, the brakes have been ok, normal for normal braking. One day had to dynamite the brakes and really had to stand on em to get more stopping power (and never got em to lockup). Not like the brakes are hard like when there's no vacuum, but it seems a tad softer than other vehicles I've driven. Been wanting to put bigger tires on this truck for better ground clearance, but if I can't lockup 225's (28"), gonna be worse with bigger tires.

Started off with a simple inspection, new front pads, vented/slotted rotors from summit, and bleed brakes. Put carbon-metallic DOT FG rated pads up front, and used high temp Wilwood DOT4 fluid to flush the system. Looking things over I could find no leaks. When bleeding/flushing the brakes, I was amazed the amount of brown rusty stuff coming out of the system. When bleeding, 1st 1/2 of pedal travel felt normal, but last 1/2 of travel seemed to take more effort, more friction. Took er out for a test drive, maybe ever so slightly better, but still not able to lockup brakes. Going downhill at 50mph standing on em I was able to get the rear to hint at lockup, dry road. I'm guessing I'm at ~3/4 to lockup from seat of pants deceleration rate. 
Had a chat with the neighbor, and he suggested I adjust the rear brakes up. Cranked em up as far as I could and still get the drums on with the rust ridge it had. Test drive, not much different.

Next project was the olds needed brakes. While putting new pads on trying to squeeze the calipers, they were very sticky. So I replaced em (and the hoses) and wow, the olds had a noticeable difference. Figured I'd try the same on the truck next time I was back in town. So I ordered stainless steel Russell brake lines from summit, picked up rebuilt rotors from autozone, and more brake fluid...   

So, got that order in and eventually put the new brake lines and calipers on, and bled the system good. Out for a test drive again, drove around normal for a little bit, then up to 50mph to test it out, no change. Needless to say by this point I was pretty frustrated.  So, drove around for a few weeks with all this new hardware.

The last suggestion I had before moving upstream to the master cylinder was to take the rust edge off the drum so that I could adjust em up further, and scuff up the pads and rotor in the case that the new rotors and pads glazed over and didn't bed correctly. So, got things all scuffed up and reassembled, and like all the other test drives, no real change. The exception is that it's now the rainy season here in Seattle, and even on wet roads I can't get the front to lockup. Even if I hit the brakes going uphill, the rears will lockup first. I haven't performed any actual brake distance measurements, but I suspect it's longer than I'd like to know. The one positive thing is that even with original brakes in back (43k orig miles), the rear brakes seem to work fine. However, it sure puts the brakes on the bigger tire upgrade for running around the woods.

So, what next?  I'm guessing the combination valve ought to be ok with all the fluid that's been flushed through it. The booster I'm guessing is OK since it does make boost. I'm guessing the master cylinder has had enough corrosion that it leaks past seals, especially beyond the normal braking stroke. Smell like I'm on the right track? Should I replace the booster anyway? If I do replace the booster, should I upgrade to the dual diaphragm JB5 booster? Any other little things I should check, adjust, or change before shelling out more bucks at the parts store?

[Irish_Alley]

I think it sounds like the master cylinder. wait for more people that know about brakes but I kind of have the same problem replaced the booster still didn’t fix it now all I think I have left is the cylinder

[N2TRUX]

  I'm guessing the combination valve ought to be ok with all the fluid that's been flushed through it.....

You might take a look at it anyway. It almost sounds as if your bias is off. Granted it should lock the rear up if it was the proportioning valve, but I wouldn't rule it out.

[1980c10]

A master cylinder is cheap probably $25-45. I would try it. Sounds like not much left to try.

[mdalbertson87]

sounds like a vacuum leak to me......... if you get pressure in the rear and nothing up front you have pressure escaping somewhere in the system, if im not mistaken,(taking a shot in the dark) your booster is bad or you need a bottle of dish soap and water and start spraying all your vacuum lines. that MIGHT be it, i dont know to much about these trucks but its only a guess!!!!! good luck

[Danbbb]

Definitely sounds like the brake booster to me.  What you can do is pull a vacuum at the little valve on the front of the booster.  If it holds a vacuum then I would check something else.  If you can't get it to hold a vacuum then replace the booster.  I just had to do this on my truck.  I felt like I had to stand on the pedal to get the truck to come to a complete stop.  After replacing the booster it stops even faster than a typical car.   Good luck!

[Dieselfume]

,
Well, none of the above, sorta... Got a new master cylinder and JB5 dual-diaphragm booster (upgrade from the JB3, figured why not while I was at it) and gave it a shot. Installed just the new master cylinder and took it for a test drive, a bit crisper on the brakes, but standing on it, couldn't lock em up. After that I was like WTH... Then I started reading the instructions that came with the vacuum booster (which were generic, but more detailed than the F&^%$$& Chiltons manual). And in there was the key tidbit, proper pedal travel and rigging. This truck had 1/2-5/8" of pedal travel before doing anything. Total travel was just over 3". Spec is 3.5" pedal travel, and <1/4" pedal slop. Also had an alternate spec of no more than 1/2"  travel before you see fluid squirt up in the reservoir. Well I was about 3/4"before something would squirt. So, when i was installing the new booster, the rod that screws onto the booster input and attaches to the pedal was too short when tighened up all the way. I added 4 lockwashers and put back together. Now nearly zero slop on the pedal before braking action. You see a squirt in the reservoir around 3/8" pedal travel, but the total travel was still a tad over 3". The test drive put a smile on my face, locks em up! level, uphill and downhill. Now i feel safe about upgrading to 33's since I've proven I've got enough brake torque to lockup 28's (225's). Only disappointing part is that I was going to go with 33x9.5-15 BFG AT for better clearances on stock ride height, and better gas mileage. However, nobody has em in stock and they are backordered until Feb 2011. So, I guess will hit the tire store this weekend for some 33x10.5 since they are available. 12.5 are too wide to fit.

[ccz145a]

Glad to hear you got it sorted out.

[Grim 82]

That's some good information there. Too bad you don't live closer, I would give you a great deal on a set of 33-10.50r15 BFG AT/KO with 10k on them. I just ordered some 12.5 wides.