Brake bleeding woes

[ChrisC]

Greetings all, need a little help as the brakes on my M37 are making me crazy.

I am close to completing the restoration of my truck, I have replaced the wheel cylinders, master, shoes, hardware and all the rubber lines. I bled and bled and bled the thing and the pedal would not come up, and there was a ton of air in the lines. I pulled all the wheels and chased all the lines, and re-checked all the fittings no signs of leaks.
I swapped in another new master, now the brakes bleed until I get about “half” a pedal, then the pedal goes to the floor, and there is a ton of air in the lines again, I go back, bleed it again, pedal comes about half way up, and then bam, to the floor, I’ve been going round and round with this thing and am at a loss.

It seems to point to a second bad master.

My other question is the brake light switch; could it be allowing air in? How do you guys seal the threads on the switch?

[Cal_Gary]

Sorry for the difficulty you're having, being so close to having your restoration complete.

I just did my brakes so everything is still fresh in my mind, so:

First, tell us that you're using a power bleeder-you're wasting your time without it (again, I'm speaking from experience). If you're pumping up and having a buddy crack the bleeders it's possible that not enough fluid is being kept in the reservoir, allowing air in (I assume that you're bleeding the furthest WC then working toward the MC which is the correct sequence);
1B-check the MC plunger where the pedal rod centers itself-sometimes the plunger area leaks into the boot and it's not seen right away;
2-Check your MC fill plug to see if it has a small hole in the side just under the seat-some do, for reasons unknown, and this could allow air in;
3-are the bolts that hold the top of the MC tight, with a good gasket?
4-is the MC top plate warped, allowing air in?
5-the brake switch could indeed be faulty, and not just the threads, but the top half where the electricals are located-could have a hair-line crack that's sucking air. If you have the same size pipe plug, try plugging it then bleeding again. I use Teflon thread tape on those threads.

I know I just gave you a laundry list to check-please keep us posted; I'm sure we'd all like to know what you find.
Thanks,
Gary

[jordankjcm]

I second Cal_Gary's comment regarding a power bleeder. I just changed all the fluid in my brake system last week and couldn't have done it without one of those gems. You can make one from directions found on the net or buy one. Not too expensive as I recall. Worth every penny I think. Make sure you are bleeding the lines in the proper order. Driver's side rear cylinder is the cylinder farthest away from the MC, even though "not as the crow flies." The line placement makes that cylinder the farthest away, followed by passenger rear cylinder. Also make sure the vent line petcock is turned off when you use the pressure bleeder otherwise you'll be pumping fluid into the vent line.

[Ray505]

Just a thought. Is the valve on the master cylinder vent open if you are manually bleeding the brakes?

[ChrisC]

The valve was open.
The first master had no pedal at all.
The second master starts to build a little pedal, then goes right to the floor.
There is no fluid in the boot.

I ordered a power bleeder, I am hoping that it will help uncover the issue.

[k8icu]

You've gone all over the system and looked for leaks right? Also just because the part is new or NOS doesn't mean it is working. Check and recheck everything. Good luck. That kind of problem can drive you crazy.

[MSeriesRebuild]

Been working on and rebuilding brake systems for many years. Some of the info offered here is basically irrelevant; such as any leak around the M/C top cover. It would have no effect at all on air in the system, in fact any air will eventually work its way back to the M/C exhausting via the vent. If there is no vent at the top of the M/C fluid reservoir, the system draws a vacuum and will not function. In fact bleeding air from a well functioning system with the top completely off the M/C makes no difference at all; as long as there is fluid in the reservoir. Air in the system issues (if there is one) will always be between the M/C outlet and the wheel cylinders. What is happening above the fluid level in the reservoir has no bearing. Of course close any ventilation valves if using a pressure bleeder so you don't fill the vent system with fluid.

Since you are attempting to bleed by pedal pumping, how you are doing that pumping is huge! If you are pushing the pedal until it bottoms, there is a high likelihood that you are ruining the piston cups in the M/C in the process, disabling its ability to pump and build pressure. You said you are using "new" M/C's. That is vague; are you getting NOS M/C from someone? If that is true, likely they are so old even in original, unopened packaging that the internals are no good. Are you using an original M/C that has been rebuilt, once again that is likely no good, especially if you are rebuilding with an NOS rebuild kit, once again parts are so old they are no good. Then there is the issue of honing the cylinder to remove pits, and out-of round conditions before installing a new kit, most times by the time the bore is honed the necessary amount to remove imperfections, the bore is oversized to the point a rebuild kit, even if it's of new manufacture will no work. Bottom line; if bleeding by manual pumping, never depress the pedal more than 3/4 of its full travel. Manual bleeding works fine if done right, and all system components are up to par; however pressure bleeding is much better. We have a top of the line pressure bleeding system, but still there are instances where that does not work for various reasons making manual bleeding the only alternative. In fact we had one recently that was that way. One other issue, the best procedure is to put an inch or so of fluid in a glass jar, connect a good hose to the W/C bleeder valve and submerge the end of it in the fluid in the jar. This way you can see air bubbles in the fluid, and air will be prevented from re-entering the system as your assistant pumps the pedal. Once fluid runs clear of bubbles, close the bleeder and move to the next bleeder in line.

My suggestion is this if success is not achieved. Remove the M/C, place in a vise, put you finger tip over the outlet and depress the piston several strokes . If no pressure builds against your finger, the issue is in the M/C. If pressure builds and forces your finger off the fitting letting fluid escape, your problem is not the M/C, but is somewhere further down the line. You will have to go through a process of elimination until you find the issue. If your M/C is up to par, any leak can be easily detected by fluid escaping. Example; if there were a leak at the stop light switch, you would see fluid seepage in that area. Wash everything thoroughly so you can detect seepage if it exist.

No mention has been made as to how well your shoe to drum clearance is adjusted. That MUST be done RIGHT or you can literally pump forever to no avail. This is a straight forward, common sense procedure, however first things first in rebuilding any system on the truck, always remember if this isn't done right, most often it also means that which is associated and done later won't function right either. All we have to go on here is info YOU HAVE PROVIDED, any response is based on that. With that said, my thinking at this point is the use of a pressure bleeder will not solve your problem, it sounds to me like there is some other undetected issue in play that has not been resolved.

[Cal_Gary]

Your expertise is again, apparent, Charles. My response was based on brainstorming any/all possibilities-thanks for the clarification.
Gary

[ChrisC]

OK, so I finally got a chance to head back out in the shop and play with the new power bleeder. Hooked it up, pumped it up and crawled around under the truck looking for signs, nothing.
So I figured I'd bleed the brakes and see what happens, everything works great, took me 10 minutes to bleed, got a pedal and everything. Took the truck out and made a bunch of laps around the parking lot, everything works good. I think that my helper was pushing the pedal down too far as mentioned in Charles' post above.

Still a bit suspicious of the thing, will go back in a few days and see if it still stops.

On the other hand, I am not sure how I lived without a power bleeder all these years! My new favorite "cake tool"
(one of my mentors from years ago called tools that you can live without but make life easier "cake tools")

[MSeriesRebuild]

OK, so I finally got a chance to head back out in the shop and play with the new power bleeder. Hooked it up, pumped it up and crawled around under the truck looking for signs, nothing.
So I figured I'd bleed the brakes and see what happens, everything works great, took me 10 minutes to bleed, got a pedal and everything. Took the truck out and made a bunch of laps around the parking lot, everything works good. I think that my helper was pushing the pedal down too far as mentioned in Charles' post above.

Still a bit suspicious of the thing, will go back in a few days and see if it still stops.

On the other hand, I am not sure how I lived without a power bleeder all these years! My new favorite "cake tool"
(one of my mentors from years ago called tools that you can live without but make life easier "cake tools")

Great, glad you got it.

[Cal_Gary]

Ditto! I had the same impression once I power-bled mine, to never do it manually again!
Gary

[pwrwagonfire]

I did mine manually, it took awhile.

Here is a trick from an ex-army mechanic who I talked to. Because the MC reservoir is so small, run oxygen tubing from the wheel cylinder bleeder to the MC. You wont have trouble with your MC going dry and binding up with air...he said that they used to do it all the time.

I've done it on 3 different trucks and worked like a charm.


-T

[ZGjethro]

If you do that on anything but a fresh fill won't you be re-introducing nasty, dark fluid back into your system instead of discarding it?

[pwrwagonfire]

If you do that on anything but a fresh fill won't you be re-introducing nasty, dark fluid back into your system instead of discarding it?

If you've got nasty dark fluid, then yes!

[ZGjethro]

Isn't that the way all brake fluid is at the caliper or drum end of the brake line?