I need to bleed my master cylinder (its ON the truck), but I'm not sure how to go about it. (Stock 1956 F600)

I figure you're to do one line at the splitter just like I would the brakes, but where I'm a bit confused is how to do this without putting air back into the MC. On the wheels you turned a bolt before releasing the pedal, the only way I see access to the MC is to where the brake lines connect, I could pump them, but how do I keep air from getting back in? Even if it's in fluid or right back into the MC, how do I keep air from going back in when I reconnect the line? Does it have something automatic or is it just in a place where the brake lines will pick it up?

Thanks

 

Give it the finger….literally………..use your finger as the stopper in a one way valve, have someone else push the brake petal down with the fitting off and hold it down, then use you finger to seal the hole and lift the petal. Repeat as necessary until air is gone, keep MC full, reconnect line with the petal up(?) Works best on the bench.

 

Don't I still risk air when I go to put the line back on though?

 

get a helper. pump brake pedal and hold, loosen banjo bolt on end of MC, make big mess spraying brake fluid, snug banjo, let up brake pedal. repeat multiple times, making sure the MC stays full. throw away or burn clothing

 

Which one in the banjo bolt?

 

Ok, Fill a Mason jar with new fluid, hook a bleeder hose to the wheel cylinder zerk, secure the other end IN the jar, when you push the pedal it will expell the air, and draw in clean fluid.

 

the banjo bolt is the one that holds the distribution block to the MC, and has the brake light switch screwed into the other end of it. it's drilled out, and has passages drilled thru by the head to let the brake fluid flow thru it. you could just loosen the brake light switch and get the same messy bleeding of the MC. they get the name from the brake line fitting they go thru as the fitting kinda looks like a banjo. a very visible example is the connection between a brake line and the brake lever/master cylinder on a motorcycle handlebar.

FYI : it's always a good idea to replace the copper banjo bolt washers when cleaning/repairing brake line fittings. cheap insurance against leaks as they're generally about a buck apiece.

 

You can't open up the brake system and bleed just parts - like the master cylinder.

You have to leave the system closed and bleed all four brake lines at the bleed fittings on the wheel cylinders that stick out of the backing plates.

You will need a helper to pump the pedal and fill the master cylinder up as the fluid level goes down.. You will need a couple litres of fluid.

Always use new fluid to fill up the mastercylinder. Do NOT refill the master cylinder withthe fluid that you have bled out of the system at the wheel cylinders.

Do the furthest wheel first - right rear. Then left rear. Next do right front then left front. I'd be tempted to go back and redo the right rear after all that, just to be sure again.

Fill the master cylinder. Puta rubber hose on the wheel cylinder bleed fitting then loosen the fitting just a little. Have the helper pump the pedal and keep the master cylinder full. Pump until all the air bubbles are gone (usually about a cup of fluid). Then close the fitting and remove the hose -move tothe next wheel.

If the master cylinder is allowed to pump dry, start all over.

 

Well ………….yes you can…………as it is recommended that you bench bleed a new master cylinder.

[QUOTE]Pump, hold down on petal, open bleeder, close bleeder, release petal / pump again, repeat

 

A friend of mine made a great tool for this a while back,.He used your everyday pint sized plastic soda bottle, found a piece of rubber vacume hose (16-18")that would fittight on the bleeder,drilled abunch of1/32" holes around the perimeter of the cap..and one larger hole in the center " just" large enough for the vacume hose would fit real snug.
Run the hose to within 1/2" of the bottom of the plastic bottle,..add 1 cup of brake fluid and screw the lid on. Attach to the brake bleeder and pump the brake pedal after loosing said bleeder.Keep master cylinder filled.
A length of mechanics wire wrapped around the bottle neck (16") will allow a person to hang the bottle from the frame/exhaust etc. I usually go to each wheel and make sure I can loosen all 4 bleeders before getting too far in this job..make sure the bleeder screw is not clogged with rust and crap,..and that fluid "will" actualy come out the bleeder. Sometimes a little heat may encourage a really old and rusted one to move. If the bleeder is blocked..it will be a messy job even with the bleeder bottle,because the fluid is not coming thru the bleeder..but from between it and the wheel cylinder.I keep one of these bottles hanging on the shop wall, real handy and it keeps the jobsite floor cleaner. Good Luck!

 

Agreed you should always bench bleed a master cylinder before you install it. Most of the master cylinders I have installed come with a ``bleeding kit``. All it is is usually a couple bleed screws and a couple inches of rubber hose. You screw the bleed screws in the master cylinder ports. Hook the hose onto the bleed screws and put the other end back in the master cylinder reservoir. Fill the reservoir with fluid and pump the master cylinder until you see no more bubbles flowing through the rubber hose.

The very small amount of air that is going to be trapped at the master cylinder ports when you connect the brake lines will then be bleed out when you bleed the system at each wheel starting farthest from the master cylinder and moving closer.

 

The line bleeding I have down, it's just the master cylinder bleeding. It's already on the truck, I'd really rather not take it off (it's NOT new).

I bleed my brakes earlier and adjusted the brake pads, but I still had a little bit of play in the brake pedal, so after checking I thought it was because I didn't bleed the MC itself.

I am thinking it's possible that there is a little bit of air stuck in that piece that runs from one hole in the splitter to another (I posted on this earlier today, I thought I could just use plugs but when I did I had ZERO brakes and a very stiff pedal without any play. I had to put it back on to get my brakes back). I'm just thinking it's possible for it to have an air bubble stuck in the middle of it, though I'm not sure how I'll get it out.


Does the MC have to be bleed if it's not new? Just seems like that might be where my problem is.

 

If you have trouble with the bleeding of the system you can also build yourself some thing like I do: You take an old cap of an master cylinder or any other cap that will fit on it. Then you doing a hole in it and place a tire valve in there. Ready now you can bleed the system with presure, you only must open wheel cylinders.(One after another of cours) Just fill up offen, it is very quickly empty.
It works good.
BUT dont take the presure to high max 1,5 bar otherwise your cylinder can get damage.

 

standard junction block has 3 lines, yours has 5. considering truck is f-600, did this have a power booster in system at one time? plugging those ports and losing all brakes says something else is going on. is master cyl clean inside, or possibly full of gunk? seeing as you are going to bleed junction anyway, why not take junction off and see if things are clean. you might be in for a surprise. replace those copper washers!! should be separate line to each front wheel and one line to the back. job lot sells these blocks. the small amount of air that might get in system when reattaching lines will be disposed of when full system is bled. (like 1957f100 said).

 

Thanks for the help.

I don't think it had a power booster, if it did it's long gone.

The inside of the MC looks very clean, course I haven't torn into it. It was a business truck for 50 years so it was serviced regularly, and had air brakes for some time which might account of my good luck with it.


I'm not sure why they effected the brakes when I plugged them. The pedal was very solid, I could move it down about an inch but it never changed (unlike the problem I'm trying to fix, if the pedal was like that and my brakes worked it be gold). I can't think of why it would effect it so. The split shouldn't be plugged, there was a noticeable difference with the pedal when I first bleed the front and back, it could be noticed when I was opening the valve (so it shouldn't be clogged, fresh fluid came out as well)



It just seems odd that there would be no brakes with that piece gone. I imagine the fluid wasn't going anywhere and thus the stiff pedal... and I'm still iffy about this piece as it seems to me if you have fluid pushing in from both sides, you can't get rid of any air stuck in the middle of the tube. Anyone have a diagram or a picture of what one of these things looks like on the inside? (Particularly a 5 way)

My brakes did work, I was trying to get rid of a little play. I had bleed the lines and adjusted the pads, but I could still gain an inch on pedal on the second pump and if I used the brakes on the first try it felt more like a whip (slow stop that quickly grabs faster and faster till it "whips")

There are a lot of things I plan to do, but right now I can't do them all and it's the only thing keeping me from driving it at this time.

I guess I'll try my luck with bleeding the MC, and try to figure out a way to use that one like without it getting air in it (maybe I need to add a 3-way connector with a valve I can access in the middle)

Thanks for the help, keep it coming as I'm still in the "hoping that will work" phase with it, lol