Studebaker US6 always get air bubbles when vacuum bleed wheel cylinders

[sixbysix]

Am having trouble with air in my US6 brake system. I can't seem to (vacuum) bleed three of the 4 back wheels without getting air bubbles. Brake pedal is going to the floor. So there seems to be an air leak somewhere I can't find. I am not losing brake fluid as far as I can tell - the master cylinder doesn't need topping up. When I press the brake pedal there doesn't seem to be any movement of brake fluid in the master cylinder (when looking at the master cylinder reservoir with the cap out).
Any suggestions?

[Joe Gopan]

Use a pressure bleeder, keep your master Cylinder full. What type vacuum bleeder are you using and how old are your brake cylinders?

[artificer]

Sometimes sucking air through the bleeder screw threads.
Marty once suggested using teflon liquid on the threads. This also stops the bleeders rusting in the wheel cylinder.
I have used a thick bead of paint around where the thread fits in the wheel cylinder.
Joel might be onto something when he asks what type of vacuum bleeder you are using....the little Mytvac units are not up to much.
Here is a surplus standard type mechanical fuel pump I use for standard & reverse bleeding.

[sixbysix]

Hi Ben
wheel cylinders were relined stainless steel about 5 years ago.
Vacuum bleeder is a Toledo that you hook up to an air compressor

I could organise a pressure bleeder I guess - use a rubber stopper to replace the master cylinder cap with a hole drilled in it and connect to an insecticide hand pump ...?

[artificer]

Are you bleeding the hydrovac?
Here's how to make a simple pressure bleeder:
http://jeepdraw.com/images/HOW%20to%20B ... LEEDER.pdf

[sixbysix]

Hi Artificer
you could possibly be onto something there have always thought that the threads might be an air "leak" point.
The thing that concerns me is that when I look at the master cylinder reservoir and pump the pedal there is zero Brake fluid movement. I also pulled out the vacuum bleeder because my attempts at a normal pump the pedal manual bleed - failed to get any brake fluid coming out of the bleeder sites - except for one ( i.e. failed for 3 of the 4 rear wheels - note I haven't tried the front wheels yet .....

[sixbysix]

Hi Artificer
Studebaker US6 doest have a Hydrovac it has a Power booster with no hydraulic fluid attachments - just vacuum lines that run ta check valve and operating valve

[artificer]

Worked on both Studebakers & GMC's when in the military & thought they both used hydrovacs. You are probably correct as memory could be fading on what one did 50 odd years ago.
Pulling air @ bleeders is only relative to when one is vacuum bleeding.

[sixbysix]

Yes i understand re the vacuum bleeding
Am just a little concerned that I may have a "shot" master cylinder - your thoughts?
This was relined in stainless around 5 years ago also .....

[artificer]

BTW do not use any NOS rubber brake components.

Very possible the primary cup has failed.
Check:

This condition will allow the pedal to go down with next to no resistance.
With a helper loosen a fitting near the master cylinder outlet & see if fluid is coming out under pressure when the pedal is depressed.
Wash off fluid if it exits @ this point.

Possible Causes:
Usually there is a thin brass shim between the piston & primary cup.
This causes the fluid rushing past the primary cup collapsing on brake release to be equally distributed around the cup.
Left out, as often occurs, the primary cup becomes eroded where the piston's little holes mate with the primary cup.
Eventually the cup looks like a soft drink bottle cap & will not produce or hold pressure.

Also pedal pump bleeding can allow the primary cup to get damaged if the piston & primary cup bottom out or come in contact with a master cylinder check valve, when they have traveled too far.

[sixbysix]

There is some resistance if the truck is not running and almost no resistance when the truck is running ......

[Joe Gopan]

Worked on both Studebakers & GMC's when in the military & thought they both used hydrovacs. You are probably correct as memory could be fading on what one did 50 odd years ago.
Pulling air @ bleeders is only relative to when one is vacuum bleeding.

Studebaker US-6 and US-6X4 did not have Hydrovacs, the used a Vacuum Power Cylinder which assisted the the Master Cylinder via a mechanical linkage of rods and levers. Your Master Cylinder is the same as the CCKW with the hydrovac. It is a WAGNER F-1251 and the Wagner kit number is a 5350.

Perhaps it is time for a Master Cylinder Kit.

[W. Winget]

How about the possibility that you are pulling air around the slave cylinder cups...they are designed to expand against the cylinder walls under pressure. Pulling a vacuum on the system will induce air pressure in the opposite direction of the cylinder cup design.
Try gravity bleeding with helper pushing or pressure from master and see if you get bubbles.
Silicone fluid will hold bubbles in suspension much longer than Dot 3 or 4 too.
V/R W. Winget

[Joe Gopan]

Assuming your brake lines and cylinders are serviceable, you may have better luck with pressure or manual bleeding. The source of your air bubbles may be from the master cylinder which is quite a distance from the rear wheel cylinders.

[sixbysix]

thanks all - will let you know what I find