M8 Greyhound brake help

[WillWagner]

Well, i'm stumped. I have read and been told that the braking system on the M8 and M20 vehicles are a biatch, but how bad could it really be , right? Well, I gotta say that they are the most difficult ones I have ever done and I have hit a wall with an issue that they have.
Rebuilt the MC..same one as a deuce, blew out all the lines and flushed them, replaced the two axle to knuckle rubber lines due to swelling internally and rebuilt the wheel cyls with updated parts. On assembly, gave the shoe clearance .010 all the way around. Rebuilt the hydraboost.
We have bled the pee pee out of them and I am positive that there is no air in the system...put a gallon through it yesterday alone. Here's the issue. If you step on the pedal, it goes to the floor, but not like it is full of air. There is definatly fluid resistance there. Let up, re apply and perfect pedal. Check this out. If you TAP the pedal, as in depress it 1/2 inch and let up..perfect pedal on the next stroke. All 6 wheels will lock upon the second application and release fine.
The pistons in the new style wheel cylinder kits are shorter than the old ones, but, once the fluid has filled the system, that shouldn't be an issue because the stroke of the MC piston will still pressurize the cylinders the same amount as with the old pistons, there is just more total volume, correct? There is freeplay in the pedal and it, the MC, is returning to the top of the stroke, no leakage from the wheel cyls.
I looked for kinked/pinched lines and saw none, that should cause slow application and release of 1 or more wheels, but it isn't doing that.
I thought that it might just need to be driven, so I took it for a spin around the museum grounds, it does the same thing. If you apply the brakes once using the pedal to the floor or the slight tap, it stops great on the 2nd application, release it, drive 50 feet, same thing, two pedal actions to stop. Any help or ideas will be greatly appreciated.

 

[Westech]

sounds like bad master...... seen it a 100 times.. a good pump/tap and your all good. I know you said you rebuilt it but it sure sounds like the cups are bad or they are flipping inside the bore.

 

[NDT]

I have been driving my M135 like that since 1983. In my case, the shoes are adjusted too far away from the drums. Make sure your shoes are just about dragging. On the M8, you may also have air in the line between the master and the booster. You may need to pressure bleed or crack a line at the high point in the hull. The first brake application compresses the bubble.

 

[Gamagoat1]

Will,
Did you do anything to the Hydrovac? I replaced mine with one out of a Halftrack, same except fpr the pilot line and you get higher pressure. It sounds like the MC is working but not the Hydrovac. I did the same as you with the pistons, the orig have an O ring around them but I replaced them with the shorter ones with the rubber cup.. Worked great. Also, remember the Hydrovac works on vacuum and not + pressure, so the engine has to be running to get the assist.
If you're interested, I can send you a copy of the TM 9-1827B that covers the Bendix Vacuum Break Systems. Covers operation and rebuild of first and second systems. Shows the one you have (with two lines coming off the MC) and the one the halftrack uses(With one line). I think SECO still has parts for these units.

 

[NDT]

G102 and G136 are not the same hydrovac, the M8's has a bigger hyd cylinder.

 

[Gamagoat1]

You are right the G-102 is not the same as the G-136, G176, however they are interchangeable. The M-20 and M-8 are the same and use a pilot line from the MC where as the G102 does not. The cyl. is a little larger on the M-8 or M-20 but the G102 has a higher pressure rating. It worked wonders on my M-20. A hard press on the pedal would throw you out the hatch.
Actually, looking at the part numbers, the slave pistons are within .010" of the same size. The travel length changes with the series and pressure/volume requirements.

The G136, G176 used a higher volume because of 6 wheel breaks. The G102 used less volume and higher pressure, 4 break drums and more weight, from 700 lb.sq.in , G136, G176 to 1100 lb.sq.in G102.

 

[WillWagner]

Thanks for all the input, keep it comming!
I isolated the M/C and Hydraboost from the rest of the system and bled that half...had a hard pedal. I hooked up the rest of the system, bled it, and the pedal action is still the same.
I did go through the hydraboost and it does work, no fluid transfer between the vacuum and fluid side as there was before the rebuild and when the pedal is pumped, you can hear the unit working.
The shoe clearance is at .010 and on some, a bit closer, just enough to drag.
The symptom gives me a gut feeling that it is in the M/C...but it has been gone through more than once. I have another one at my house that is a good rebuild candidate and a NOS military kit. Maybe I will try that?
Please keep your thoughts comming, this needs to be up and running with paint, by the 16th of May for the Playboy Mansion/Wounded Warrior event
Thanks for all the input!
Will

 

[Gamagoat1]

Will.
Is it the stock MC. Just thinking the size of the cyl. might be different causing a volume change. Are the springs installed between the break cyls cups??

 

[NDT]

I think you still have bubbles in some of the 12 wheel cylinders. Are you using a pressure bleeder? That might do the trick.

 

[Gamagoat1]

Good point,
You did bleed em all didn't you?
A pressure bleeder would sure help.
Breakes on a M-20 are almost as fun to bleed as the accelorator hyd. system.

 

[WillWagner]

The M/C on it is the exact same one as a deuce...don't know if it the stock one, I asked Craig to check it against the unmolested M20 in the back 40. IIRC, it is a1 3/4 diameter unit. It has been bled with a pressure bleeder, multiple times and the system split in half and bled.
The springs are installed in the wheel cyls.
Thankfully, the throttle has been converted to a Morse cable , but the clutch is still hydraulic..i'm not touchin' that with a 10 foot pole!
Keep it comming guys, I gotta get this figured out!
Thanks

 

[Gamagoat1]

When Craig checks the MC have him check for two lines coming out of it. A large line at the end where you'd expect one, and a, somewhat smaller one, off the side.
If the MC has been changed and the Hydrovac is original, that's the problem. The early series 2 Hydrovac needed that pilot line to tell the vacuum valve to apply a vacuum to the vacuum piston, which will move the slave cyl. causing high pressure break fluid to be sent to the wheel cyls. With out it you get partial or no activation of the slave cylinder.

 

[WillWagner]

Gamagoat1, I can tell you that the second line is NOT there. It has a type 2 hydraboost. . I will take some pics on Tuesday and post them up. Maybe this will fix my delema. Thanks guys!

 

[WillWagner]

Here's some pics of the brake stuff.
The first 4 are of the stuff in the M8 in question. The next two are of the MC and hydrovac of an un touched M20 showing the two line MC. The hydrovac on the M8 is larger in diameter than the one on the M20. The last few are of the "touched up" M8 we are working on. I also noted a strande thing this afternoon. After the pedal is depressed and there is a good hard pedal, when it is released, there is the sound od fluid comming back into the MC?? Never heard that before. Any ideas?

 

[WillWagner]

Woops! Didn't mean to put the V100 in there!

 

[Gamagoat1]

The Hydrovac in the M-8 is the single line series two . So, it's set up right right. That's the same setup as the !/2 track. The one in the M-20 is the early series two with two lines, smaller, with less psi to the wheel cyls.
The only thing I can say is that the Hydrovac is stuck at the vacuum valve. Will, if you'll PM me with an address I'll send you a copy of the TM 9-1827B covering the Hydrovac. It's a crapy copy but easier to read than LOGSA manuals.
Let me know.
If it stops on the second push of the pedal maybe it can be used that way untill a replacement can be found.
As you know, the fluid returns to the MC when you release the pedal, never heard it tho, maybe it's because it's inclosed in the hull like it is. Can't imagine that it means any thing wrong.

 

[WillWagner]

It was just brought to my attention that I did not update this to let people know what fixed the pig! I do believe that it was air in the system as NDT said. I ended up using an oil can filled with brake fluid and reverse power bled the system from the wheel cylinders back up to the MC. Messy, time consuming, but it worked!

Only took 11 years to update this! That's worse than the M106 cooling fan!

 

[D!rk]

It was just brought to my attention that I did not update this to let people know what fixed the pig! I do believe that it was air in the system as NDT said. I ended up using an oil can filled with brake fluid and reverse power bled the system from the wheel cylinders back up to the MC. Messy, time consuming, but it worked!

Only took 11 years to update this! That's worse than the M106 cooling fan!

Will, i have the same problem with te brakes of the M8 here in Belgium , first push on brake padel to the bottom, second push already some resitance on the brake padel. bled the system with presure from MC to Hydrovak, only olie, no air. There have been trobbels with hydrovac before, after the push on te brake padel, brakes blockt, dit not come lose again. Can you give my more info how you solved your problem ? If there is resistance on te padel, if is not feellling sponsee. Sorry for te bad riding, i m learning it.

 

[WillWagner]

Your issue now is no pedal on the first push and a good pedal after that, or the brakes stay applied after releasing the pedal?

 

[D!rk]

Your issue now is no pedal on the first push and a good pedal after that, or the brakes stay applied after releasing the pedal?

The brakes stay not on after releasing te pedal, that was te issue before de booster was rebuild en tested in one testbench, brakes work as normal, only there is no pressure by te first pedal stroke , brekes have been purged thil cleen oil comes out of bleeder on booster en wheels .