62MackB85FT Posted May 8, 2019 Share Posted May 8, 2019 Brakes go to floor initially, then as pumped the pedal stays higher and firmer to the point brake feels good. If pressure is kept on pedal, brakes stay firm and pedal high. But once pedal let go, 20 seconds later pedal goes to floor again until pumped. Plenty of fluid, did system bleed including two on hydrovac (tandem piston cylinder style). Lines look good, not losing fluid. Any ideas why brake is soft before pumped? Quote Link to comment https://www.bigmacktrucks.com/topic/57098-hydrovac-brake-1962-mack-b85/ Share on other sites More sharing options...
fuzzy buzzard Posted May 8, 2019 Share Posted May 8, 2019 Did you bleed system before starting engine? I have heard bad tales about bleeding while running, so I always left engine off till all the air was gone. A vacuum pump is right handy for this job. Quote Link to comment https://www.bigmacktrucks.com/topic/57098-hydrovac-brake-1962-mack-b85/#findComment-422178 Share on other sites More sharing options...
62MackB85FT Posted May 8, 2019 Author Share Posted May 8, 2019 Yep, bleed was with engine off Quote Link to comment https://www.bigmacktrucks.com/topic/57098-hydrovac-brake-1962-mack-b85/#findComment-422180 Share on other sites More sharing options...
41chevy Posted May 8, 2019 Share Posted May 8, 2019 Lots of air gets trapped in those systems, in all reality vacuum bleeding it the best and quickest way. Otherwise you just need to spend a lot of time going back to bleed it. Air is usually in the secondary part of the booster system. Quote "OPERTUNITY IS MISSED BY MOST PEOPLE BECAUSE IT IS DRESSED IN OVERALLS AND LOOKS LIKE WORK" Thomas Edison “Life’s journey is not to arrive at the grave safely, in a well preserved body, but rather to skid in sideways, totally worn out, shouting ‘Holy shit, what a ride!’” P.T.CHESHIRE Link to comment https://www.bigmacktrucks.com/topic/57098-hydrovac-brake-1962-mack-b85/#findComment-422183 Share on other sites More sharing options...
JoeH Posted May 8, 2019 Share Posted May 8, 2019 On pickup trucks, if I replace the line along the frame heading to the rear axle I've found I usually have to pump some fluid through it before hooking it up to the rear axle line, otherwise it holds too much air and doesn't like to bleed for me. Likely because for a while I didn't have all the right tools. Now I have that cup with the tube that hooks to the bleeder valve. That thing is real handy for bleeding brakes. Just don't let brake fluid get on paint, it eats through and destroys paint pretty quick. Quote Link to comment https://www.bigmacktrucks.com/topic/57098-hydrovac-brake-1962-mack-b85/#findComment-422188 Share on other sites More sharing options...
Rob Posted May 8, 2019 Share Posted May 8, 2019 I've always filled all reservoirs on an empty system and bleed the master first to the inlet of the hydravac unit. Then bleed hydravac unit, then move to the wheel ends. Trying to bleed the system as a single unit leaves too many variables and guessing. Break the overall system into components and work it from that angle. Vacuum is much easier if you have access to the process but it goes pretty quick with a couple of folks working in unison. Quote Plodding along with no job nor practical application for my existence, but still trying to fix what's broke. Link to comment https://www.bigmacktrucks.com/topic/57098-hydrovac-brake-1962-mack-b85/#findComment-422190 Share on other sites More sharing options...
62MackB85FT Posted May 8, 2019 Author Share Posted May 8, 2019 Did bleed per manual in correct order. Did it manually and it does recommend pressure bleeding. Thinking of picking up a motive power bleeder and giving a try Quote Link to comment https://www.bigmacktrucks.com/topic/57098-hydrovac-brake-1962-mack-b85/#findComment-422205 Share on other sites More sharing options...
62MackB85FT Posted May 10, 2019 Author Share Posted May 10, 2019 Leak in master, once past 10 psi on power bleeder the master cylinder started leaking. The more pressure the worse it got. Guess I’ll be looking for a new one or trying to rebuild it. Quote Link to comment https://www.bigmacktrucks.com/topic/57098-hydrovac-brake-1962-mack-b85/#findComment-422381 Share on other sites More sharing options...
62MackB85FT Posted May 16, 2019 Author Share Posted May 16, 2019 If anyone looking for master and there’s is a wagner casting, give them a call. They looked up truck and were able to find what master cylinder needed. Turned out to be MC2842, still in production and readily available all over online. Never been the case on this rig in last 5 years, guess first for everything Quote Link to comment https://www.bigmacktrucks.com/topic/57098-hydrovac-brake-1962-mack-b85/#findComment-422921 Share on other sites More sharing options...
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