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bobkyllo 
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Joined: 14 Sep 2009
Location: minnesota
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 Topic: 74 ford f600
Posted: 06 Aug 2018 at 10:57pm |
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My neighbor talked me into helping him with his brakes on his 74 ford f600 grain truck.
We've bleed the brakes twice now. He has put a new brake booster on. Also the master cylinder looks to be new say 4 or 5 years ago. It still has the shiny machined/cast look to it.
Anyway the problem is the brakes seem to fade away. Going down the road you stomp on the pedal like it's an emergency stop and you stop. They work very well except when you sit with your foot on the brake pedal then the pedal goes from about half way to the floor.
So say you stop for example the pedal will go half way down which is normal but then if you just sit there with your foot on the brake like your waiting for traffic the pedal will slowly fall to the floor.
A couple times tonight it seemed like the pedal was farther down then normal. But when you release and press again it was normal.
We drove around for probably half hour doing lots of stopping. Got home and the fluid was not down. Also split the master cylinder and booster and no sign of fluid going in there.
What do you guys think. Also I jacked up one wheel and done some testing. The wheel I picked, when spun by hand does stop great. Also it holds even when the pedal falls to the floor
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shameless dude
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Joined: 10 Apr 2017
Location: east NE
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Posted: 07 Aug 2018 at 12:23am |
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master cyl or leak in line somewhere
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bobkyllo
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Posted: 07 Aug 2018 at 3:21am |
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I'm pretty certain there is no line leak. As much as we screwed around with it the level should have gone down some and it didn't.
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DiyDave
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Location: Gambrills, MD
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Posted: 07 Aug 2018 at 4:40am |
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master cyl seals leaking...
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jaybmiller
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Posted: 07 Aug 2018 at 6:03am |
either 1) the master cylinder has bad rubbers ,so fluid recirculates within master cylinder
OR
2) you've got a slug of air in one of the lines
My vote is for #2, have that with forklift as it's 'fun' to TRY to bleed. I'd grab a 2nd set of hands and bleed them brakes again, starting with the furthest line(?). you'll need a lot of fluid. I'd say the line is fine when you can 1/2 fill a coffee cup . In my case I've got a bent line that's trapped a slug of air. great brakes if you pump them(locks up left wheel),but pedal to the floor first time. I need to rework the 'feed' line( master to Tee) to get rid of the 'hump'. Hopefully that way, the air will come back to master(self bleeding ). Unlike your truck, I ain't got ANY headroom under the forklift !
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bobkyllo
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Posted: 07 Aug 2018 at 7:15am |
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Wouldn't air show up right away though not just after holding the brake for a bit? Also the varrying pedal height makes me think it's something other then air
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LouSWPA
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Posted: 07 Aug 2018 at 7:24am |
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the master cylinder is leaking internally
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Stan IL&TN
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Posted: 07 Aug 2018 at 7:44am |
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If no leak is found I would bleed it one more time and if no improvement then consider a new master cylinder.
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Roger (NE)
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Posted: 07 Aug 2018 at 10:03am |
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The Fords of that era in the bigger series (F600 etc) were notorious for retaining air in the lines. Keep bleeding and eventually it will usually work it's way out of there. I completely redid the brakes on my LN600 a number of years ago and had zero problems when I put it back together but had a neighbor that did his F700 dump truck and it took multiple bleedings over a period of several weeks to get all the air out. As long as you feel safe with it that way, drive it and then bleed...over and over. Doesn't seem to be any logical explanation as to why some are an easy fix and others hard.
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Les Royer
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Posted: 07 Aug 2018 at 8:40pm |
Use a pressure bleeder. Pumping the pedal and cracking the line won't do it no mater how many years you try. The problem is while youre having that cup of coffee in between when yer saying ok, pump it up again, ok ready! that air is going back up where it came from and yer pushing it nowhere. A constant pressure bleed will be what you need to do the trick. It takes a constant flow to get it to move.
But I'm leaning towards the master cylinder. Cause it's easier to buy one of those than it is to make a bleeder pot.
Don't forget the bleeder at the booster. There should be one there too to bleed, if it's one of those that's mounted on the frame rail.
Edited by Les Royer - 07 Aug 2018 at 8:46pm
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Brian Jasper co. Ia
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Posted: 07 Aug 2018 at 8:46pm |
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Years ago when I worked at a farm store one of their trucks had a brake line rust and break. I replaced the broken line and bled and bled and never did get the air out. It was one of those Hydro Vac systems. We ended up sending it to a truck shop and like Les said, they used a pressure bleeder and it was fine.
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"Any man who thinks he can be happy and prosperous by letting the government take care of him better take a closer look at the American Indian." Henry Ford
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HD6GTOM
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Posted: 07 Aug 2018 at 9:00pm |
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I owned a couple of those. As stated the master cylinder is bypassing internally. Buy a new one, don't try to rebuild the old one. Experience speaking here.
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KJCHRIS
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Posted: 07 Aug 2018 at 9:45pm |
My opinion, take what JayBMiller and Les Royer said put them together and you'll get them working. Has Either fluid by-passing in MC or Booster or an air slug still in it. Pressure bleeder is almost a must have on the mid 60-early 80's MD trucks if the booster is the style not directly attached to the MC.
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