To Bench Prime Or Not To Bench Prime?

[offshores]

I’m converting to a dual master cylinder on a ’65 Bug and have been reading up on the bleeding process. I’ve read where I should bench bleed or prime first and then I’ve read where it’s not necessary. I decided I would attempt to bench prime and then noticed my MC didn’t come with any plugs or a “priming kit” that was mentioned in another article. What are my options? Everything is brand new, hard and soft lines, shoes, wheel cyls, master, everything. I’m seriously just going in circles reading all the posts on here (by means of using the search function) and thought it would suit me best to just ask. Thanks!!!

[DuaneL]

you are not going to get a defining answer lots of us don't bench bleed and have no problem others swear that it has to be bench bled. some will just loosen the hard lines at the master while some one presses the brake pedal to achieve the same results as bench bleeding.
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1971 std bug, 1776, ACN Super Stock 34-3, dvda dist, 27x8.50 14 tires

[Helfen]

I find it can be messy and I have not had trouble just doing it on the car. I just had a look at my brake course handbook I got when I was attending VW of America school ( dated 1968 ). It does not mention bench bleeding, however it does say to start with the wheel furthest away from the master cylinder on single circuit brake systems and with dual circuit to start with the front wheels first and the wheel farthest away from the master ( in LH steering would be rt ft.) then the rt rear first then LR, but only after the front is bled. There is instances where it not only helpful, but a necessity to bench bleed. Cars with a proportioning valve, and cars with ABS it is a must to bench bleed or you could be there forever.

[Eric&Barb]

If you need to bench bleed, all you need to do is open a bleeder valve and have your helper press down on the pedal. Then you close off the valve and have your helper let up the pedal. Repeat till you get some fluid to burb out.

With dual circuit do the front first and then bleed. Repeat with the rear afterward.
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[offshores]

Okay, so should I open and close the lines at the mc to prime it as per DuaneL’s advice and then open a bleeder valve and have my helper press down on the pedal, etc as per Eric&Barb’s? Or should I just go straight to opening a bleeder and having a helper push on the pedal, etc (E&B’s way)? Helfen, I just read about the different bleeding order yesterday for the first time and would have had no idea. I always thought that the order started with the wheel furthest from the mc. That’s good info to know when converting over to a dual circuit. Body's off so opening the lines at the mc and applying pedal pressure would be very easy. Thanks for the help guys.

[Eric&Barb]

Dual circuit also needs to lock up all the brake shoe adjusting stars till the shoes are fully against the drums. Otherwise it is a lot harder to bleed the system.
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[Helfen]

[demon1018]

if all the lines and brake cyl are new and empty then it really wont matter if you bench bleed . if your lines are already full of fluid and you dont bench bleed , youll be pumping all that air in the system so it will take longer. they sell the kits to bench bleed at the auto part store. either way youll get it done

[pwmcguire]

I used the harbor freight vacume brake bleeder that you hook to your compressor on a brand new brake system. It worked like a dream. Best pedal I have ever had in my beetles.

[BruceJ]

[Eric&Barb]

You could do it either way.

If you have problem getting fluid out of the MC then you can "bench bleed" it while in the VW.
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[slalombuggy]

I've always started at the back right and worked to the closest wheel to the master.

The problem with bench bleeding a master is it's very messy and the few times i tried it, the fluid all out on the install anyways.

If you're replacing an existing master, there's nothing wrong with pumping a bunch of air and clean fluid through your brake system. It flushes it out and gets rid of all the old water and debris laden fluid.

I've started using DOT 5 silicone based fluids in my ACVWs it doesn't absorb water which is good as my cars sit a lot over the winter. If doing this on an existing system you have to flush the old fluid out completely as it doesn't mix with other DOT brake fluids.

brad

[BruceJ]

Okay, I think round #1 went okay. Before I started, I ran the star adjusters all the way out, so the shoes were tight against the drums, as recommended above. Then, with a helper, I bled both circuits of the MC (front first, then back), then bled the wheel cylinders RF, LF, RR, LR.

I'm going to call it a success (so far) because:

1. The entire system is all new -- dual circuit MC, WC's, drums, shoes, hard/soft lines, etc., and I found no leaks. Good clear fluid at all four. (Yay!); and
2. The brake pedal no longer goes all the way to the floor. Instead, it goes almost to the floor, but is quite firm where is stops.

Obviously, I'm not comfy with this last point, though it's progress. Assuming that the little pin behind the brake pedal, where it runs to the piston, is adjusted properly, what next? I assume another round of bleeding, at least at the wheels, is in order. Should I check to make sure the shoes are still flush, like maybe they "centered" themselves under pressure? Also, am I correct in leaving the parking brake cables slack for now?

Just trying not to waste my "helper's" time, since I'm sleeping with her .

[fluxcap]

Check all your star adjusters again. I know you already adjusted them out before you started bleeding, but since everything was brand new, the shoes may have settled some more during the pedal pumping/bleeding process. A small adjustment of the shoes can sometimes make a huge difference in pedal travel. Not sure that'll fix the pedal issue, but something to try.
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Eric - 1966 camper bus

"It's like, how much more black could this be, and the answer is none.......none more black."

[Eric&Barb]

How much play do you have between the pedal push rod and the MC piston???
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[BruceJ]

As far as I can tell, about 1mm. I noticed it was way off when I swapped the old German single-circuit to the new aftermarket dual-circuit master (or maybe it was out of spec before this). Not having an accurate way to measure 1mm down there, I adjusted as per Muir; i.e., turn rod until it just touches piston, then back off half turn, then tighten lock nut.

At least, I assume that's what I can feel in there -- the pin just touching the piston. It's like, there's no resistance, then after a very small amount of pressure with one finger, it's like -- tink -- then I feel like I'm moving something more than just the pedal. Then it travels almost, but not quite, to the floor before feeling firm.

I will check all of this again -- the adjusting stars, the free play, the bleeding. Mainly, I was wondering if my results after the first go-round was about normal for a newbie with an entirely new brake system.
Thanks

[jzjames]

havent read all the blah blah, but bench bleeding? Yeah, why not. If youve ever done it, its no biggie, and to me, is just an added benefit.

[BruceJ]

I went ahead and re-adjusted the star nuts so that the shoes are tight against the drums. Pedal play about the same -- firm, but only and inch or so from the floor. Triple-checked MC push rod and looks fine to me. Will do round #2 of the bleeding process tomorrow.

Here's a thought I'm having: Many sources (including the Bug-Me video) show the MC bleed process as having the assistant press and hold the pedal, while you crack open the output line at the MC, let air burble out, tighten nut, and repeat until there are no more bubbles.

Sounds simple, but: supposing the brake lines themselves are still loaded with air, wouldn't you just be sucking all of that air right back into the MC when the helper releases the pedal? Elsewhere in here I read an alternate method where you detach the brake line completely from the MC, and just block the hole with your thumb (carefully, so as not to get a faceful of fluid, not to mention ruined paint!). Air burbles out during compression; upon release, the port is blocked by your thumb thus ensuring you're drawing just fluid from the reservoir and not air from the brake lines. Makes more sense to me, but what do I know?

I know for most of you this isn't rocket science, but this is new territory for me, and I like to know not only what do do, but why.

[Eric&Barb]

If you shut off the bleeder valve before having your helper let up on the pedal, the MC can not suck air back into it from the brake lines.
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[johnnypan]

gravity bleed,no helper,no bench bleeding...just follow the method for dual master brake bleeding,crack the bleeder and wait..when fluid runs out its done..