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Old 05-23-2018, 02:14 PM   #1
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Need help with hydrovac system 68 intl bus

I've got a 68 international bus that has soft brakes. If you pump them once or twice it does stop, I have had this bus a couple years and just now getting around to fooling with the brakes.

I did find out (from here) i needed a power bleeder, which i did pick up, and i found a guide on how to bleed them on the net.

It says to bleed the front hydrovac unit first, which i did, pulled a vacuum on the front bleeder (on the unit itself) and cracked the bleeder. I was able to pull a little air out of the system this all went fine.

When I got to the rear unit however, i pulled a vacuum and it held, then i opened the bleed screw. I got a significant amount of air out of the system however when I went to close the bleed screw it wouldn't tighten down. Off I go to a truck parts place that deals in heavy truck parts as well as napa and found a similar bleed screw with a slightly different taper.

It threaded in and tightened right down, however it does not want to hold much of a vacuum with the bleeder closed. I removed the new bleeder screw and checked and I can see a definite ring where it has seated down inside the unit so I feel like it is cutting off the flow of fluid.

After fiddling with it a bit I moved to the rear brakes they hold a vacuum with the screw closed and i was pulling some air from the rear lines... Mostly all air very little fluid from either rear drum.

Thoughts? Is my rear unit shot letting air in? Could the difference in taper be allowing air into the hydrovac unit?
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Old 05-23-2018, 03:19 PM   #2
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Okay upon further research perhaps im getting air around the bleeder screw threads when its open, im going to try teflon tape on it and see if it'll hold a vacuum when open.

And according to the manual I found on hydrovac systems theres a check valve that has to be actuated via the pedal to get fluid into the lines from the master cyl. So after pumping the brakes and letting the pedal quickly return to top a couple times, i do seem to get much more fluid at the rear wheels now than before. I think i'll wait until I have some help pumping / adding fluid etc before continuing because running all over/under the bus trying to bleed the system solo is kind of a pain in the butt!

Also FWIW my bleeder screws on the rear drums are on the bottom of the brake assembly, I have seen people put disk brakes on the wrong side and if the bleeder screw wasn't on top it would be impossible to bleed. Is the same true for drums?
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Old 05-24-2018, 08:46 PM   #3
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Okay so, I changed out the loose bleeder screw and put some Teflon tape on the threads, enlisted to friends to help, one to keep the resivior full and another to pump the brakes. Turns out the hydrovac system has a check valve that prevents fluid from the master from going to the wheels unless itís forced In from the pedal.

Bled everything out including the hydrovac units and Iíve got brakes! Donít know how great brakes are supposed to be on her but it definitely stops well once the vac system kicks in itíll put you in the dash!

It takes a long time to bleed them out so have plenty of fluid and help, a mighty vac is a friend because you can really see when youíve evacuated the air.

And this link has a full breakdown of the brake system on the old bus.
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Old 06-01-2018, 04:26 PM   #4
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Location: SW New Hampshire
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Always happy to help.
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