73 Hi Boy brake problem

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Jun 22, 2009 | 08:23 PM

3406pack

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73 Hi Boy brake problem

Hello everyone. I am having a problem with my brakes once again. As in my previous posts,I am not getting any rear brakes but the front lock up.I have replaced the MC twice, bench bleeding both times. Previously I found that I was missing the adjusting stub in the end of the pushrod coming from the brake booster and I installed a replacement. Bled the brakes and everything was fine. Only problem was pedal wouldnt come all the way up so I installed a spring to keep my brake lights off. So im tooling down the road one day last week and SURPRIZE no brakes.(well some but still high on the pucker scale!!!!) I am at my wits end with only one thread left. Is there anyone out there with apicture of what the original MC and brake booster looks like and can u still get them reasonable. Anyone have the measurements of both pushrods? MC and BB? Any help would be greatly appreciated.

Jun 22, 2009 | 09:02 PM

75F350

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How is the brake fluid level right now? Is it low, and you see no signs of leaks? My guess is that you have a bad or rusty plunger and you have torn up the seal between the master and the booster. Vacuum is pulling the fluid out of the master, and then your brakes fail.
If you have them adjusted properly, then it should not change. So, assuming that you have everything together correctly, then you have to look for less than obvious solutions.
Many of these plungers can be rusted and pitted to thepoint where the seal at the booster will fail.
I think this migh be your trouble. I have had a similar situation and had to use some heat shrink tubing to create a reasonable sealing surface on the plunger. Yup, good old field fix, and it worked well enough to get me home so I could replace it.
Take a look for anything crusty, and see if you have any fluid left.
Might just be a bad prop valve or something, but a loss of fluid is the first thing that came to mind.

Jun 22, 2009 | 09:25 PM

3406pack

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I have just replaced the MC with a new one not reman. On the old MC which was only 8 months old with maybe 100 miles of driving, I did pull a trailer when the brakes went out from a loss of fluid but the fluid was coming out of the res cap like it was pressurizing the res instead of the brake lines. Booster was dry wheels dry everything dry except MC which had drips along the bottom with a nice coat of fluid all over. I refilled the res and started bench bleeding.got all the bubbles out and hooked the lines up to bleed the wheel cyl. To my surprise no fluid except a few drips. My proportioning valve has no button to press so I pull the line from the MC and tell the better half to push and no fluid coming from hole at the MC so now I have another new MC installed and was thinking what if someone just thru this junk together so it can be driven by a buyer. It was already missing the length adjuster on the booster rod, what else could be missing? My brake light switch is as deep as it can go and a spring still has to pull it back a full inch to touch the switch. I was hoping to get a reference to look at. Thank you for your response