Bleeding the brakes

[murfman55]

I'm trying to bleed the brakes on my scout II, theres all new lines and parts except for the old master cylinder and brake booster. I have bled a brake line before so I under stand the basics of it but never completley from the start with an empty system. I read in the knowledge base about bleeding brakes and I don't really understand the part about bench bleeding the master cylinder. Does it have to be done off the truck? Do I need the kit from autozone it talks about? Thanks Keith.

[Jay Tabor]

IH

the reason for bench bleeding or kitchen table bleeding or anywhere but on the vehicle bleeding, is so the master cylinder pistons can be bottomed out in each half of the master. and when its attached in the vehicle onto the booster [or without a booster ] the person attempting to bleed the master cyl. may not be able to completely bottom each piston in its bore. because they dont understand how the system works, and/ or the brake pedal/linkage wont allow the master cylinders pistons to be bottomed- not enough stroke.
the reason for bottoming each piston in its bore is to remove all the air..

now- why did you replace every other part of the brake system but not the master cylinder?
the master cylinder is the most important part of the system!
so now that you know, youll be going to get a kit to bleed the master cylinder..
and you dont have to buy it from autozone..
if youre smart youll find the correct size steel brake lines with correct size fittings, to match the size ports on your master cylinder.
and use them to bleed your master.
the master cylinder bleed kits are plastic fittings and hoses.
they leak, youll put more air into the system using the cheap autozone stuff,
than spending the money for two short steel brake lines and using them..

as for filling and bleeding the empty hydraulic system, just fill the master, and open the bleed valves at each wheel, when its dripping steady, then close each/all bleed valves and then go about bleeding air at/from each particular wheel.
and since you got nothing but air in the system start at the closest wheel for each half of the braking hydraulic system when you start pressure bleeding it.
and use the best hydraulic brake fluid- which is the castrol fluid, next is the valvoline synthetic, both of these have the HIGHEST WET BOILING POINT ratings.

dont forget to bleed at the combination brake valve manifold!!

[Bill usn-1]

I found that normal bleeding doesn't always work to bleed the MC.
Those have to be done opposit of the wheel cylinders.
Crack a line loose from the MC and have someone slowly depress the pedal.
Close the line just before or when the pedal bottoms.
When closed then let up on the pedal and do it again untill solid fluid comes out. Then move to the other line.

Once the MC is bled then the normal pump 3 times and hold then crack the wheel bleeder.
Close before letting up.

[Bill Worden]

Here's another tip from Neanderthal Garage: When bleeding the master cylinder, tap on the cylinder with a hammer between cycles. The vibration will help the bubbles of air migrate up into the reservoir.

[murfman55]

So leaving everything attached I am able to get fluid out of the front line on the master cylinder but I am unable to get anything out of the line closest to the firewall. It was really clean before it came of the parts scout but could there be an obstruction? Is it worth messing with, there not that expensive I guess. The only real reason I'm trying so hard with whats on there is as soon as the brakes work I can take it for a spin. Thanks again Keith.

[Jay Tabor]

YOU MUST NOT PAY ATTENTION..
the pistons in the master cylinder HAVE TO BE BOTTOMED OUT IN THEIR BORES.
and as you found out and as I said -THATS WHY ITS DONE OFF THE VEHICLE..

now you can approximate bottoming the pistons, [with master cyl still on the vehicle]
IF YOU FOLLOW THESE INSTRUCTIONS>>>>>>>>
have some one push on the brake pedal and push down as hard as they can, and hold it down. [
DID I STATE STOMP ON THE PEDAL?
DID I STATE STOMP ON IT THREE TIMES IN A ROW?
GENTLY BUT FIRMLY APPLY FOOT TO THE BRAKE PEDAL AND PUSH DOWN HARD..
SIMPLE- IT REALLY IS . .

you - unscrew a line nut on the master [in this case the front line nut] let the fluid squirt out, then tighten the line nut. YOU JUST BOTTOMED THE PISTON IN ITS BORE YAYYYYYYYY!!
DID I MENTION LETTING UP ON THE BRAKE PEDAL? NO I DIDNT]
SO WHILE the brake pedal is still being held down............ got that?
unscrew the rear line nut [usually a half a turn is all it takes] and let the fluid squirt out.
tighten the line nut.
ok NOW YOU CAN LET UP ON THE BRAKE PEDAL.
do this in that order at least 3 consecutive times, to get all [ or most] of the air out of the master.
the two pistons in the master are tandem situated- one in front of the other, now since fluid doesnt compress and you dont have any air in the front portion [piston] of the master cylinder, how can the piston be pushed tottally to the bottom of its bore?
[IT CANT UNLESS YOU OPEN THE LINE NUT].
, and thus the rear piston wont be bottomed either in the bore-thus compressing what little air is in there and force it out when the line nuts are loosened.

SO lets go over this again . .
one piston is in front of the other....
both pistons need to be bled [one after the other] to let the fluid out and bottom each of the pistons in their bore.
fluid doesnt compress
air compresses


after 3 consecutive bleeding of each brake line at master, you should be getting similar amounts of fluid- with no air, squirting past the line nut.

if the rear portion of the master cyl is the smaller reservoir- thats for the back brakes, and it helps if the brake shoes are adjusted up tight, so the brake pedal will have less travel before resistance. then the pistons move less fluid- greater pressure can be obtained and more air can be squirted out.
and after you finish bleeding the master, then you have to go around and rebleed at each wheel ,, again . . since youve been pumping air into the system all this time . . .

[Eric VanBuren]

If you can't get fluid from the port closest to the firewall and you do get fluid from the one away from it the MC is bad and needs to be replaced. DO NOT waist your time with a rebuilt one, get a new one from Napa or Carquest.

The reason for bench bleeding had nothing to do with being able to bottom out the pistons and everything to do with the orientation of the master cyl once installed. Some vehicles are not oriented so that all the air can be removed so the general recommendation is to bench bleed all MC before install.

You do not need a kit to bench bleed a MC though some MCs come with one in the package. The cheap kit will often prevent you from properly bleeding them. The best way to do it is to partially fill with fluid remove the plugs and set it so the ports are pointed upward slightly. Let gravity start the flow. Then cover the ports with your fingers and stroke the cyl. Let the fluid flow out and then plug with your finger before you release. Do this until you don't get anymore air.


When bleeding a totally dry system the procedure is different. Open the bleeder closest to the MC and have your helper press the pedal to the floor. Once the pedal reaches the floor close the bleeder and have them release the pedal. Repeat at that wheel until you get fluid then move on to the next closest wheel. DO NOT let them pump the pedal, it's a good way to destroy the new MC. Once you have fluid from each bleeder then you can switch to the pump 3 times and open the bleeder method.

[murfman55]

Thanks for all the help, I truly appreciate it. It seems shot so I'll get a new one.

[Doc Stewart]

"use the best hydraulic brake fluid- which is the castrol fluid, next is the valvoline synthetic, both of these have the HIGHEST WET BOILING POINT ratings."

Consider using DOT5 'synthetic' or 'Silicone' brake fluid in your new system. Boiling point is higher than those mentioned. It does not acquire water which rusts your system internally as DOT 3 or 4 does. If you spill it on your paint it does not ruin the paint job. All the high end restorations use DOT5. It is more expensive but it never needs to be changed. DOT3 or 4 is supposed to be changed yearly but nobody does...

[Thomas]

Originally Posted by Doc Stewart

Doc Stewart
"Tough Old Geezer"

---Any old tales of DOT 1 or 2 or do we have to look for Mr. Hofstetter to tell us one?

---As Doc stated, that DOT3 was primarily for brake drums, in the new-age muscle cars with high temp engine compartments. When we started getting into front rotors, they produced more heat, which required an even higher boiling point, which brought the need for DOT4. DOT5 was required for all-disc brakes. DOT5 was not too readily available when I last changed my brake fluid (yes, I'm guilty ), so synthetic Valvoline "exceeds DOT3&4" was the best alternative.

---Since DOT3&4 is polyethylene glycol based and DOT5 is silicone based, to use DOT5, you must completely* flush the system. You cannot mix the two. DOT5.1 is polyethylene glycol based like 3 & 4.

---AFAIK and FWIW/BTW, Motul rf600 is the best I have found, but it carries a price that says it's "the best". 600°!