Brake Bleeding Question

[MCDenny]

First time bleeding brakes (on anything)on my 97 R1100RT. Just did the rear with a Mity-Vac and something seemed wrong. Most of the flow from the bleeder nipple was air bubbles. I assumed this was air leaking past the bleeder threads, going around the corner and coming out the bleeder hole with the fluid. It did suck through three master cylinders full of new fluid.

 

My concern is this: Even with the bleeder torqued down to 'real tight' the Mity Vac still pulls a trickle of air bubbles which are clearly coming out of the hole in the bleeder. Seems like that shouldn't be. Brake pedal is hard and I can't see any leaks... but where could that air be coming from?

 

PS: Brakes worked fine before I started.

 

Thanks.

[Wooster]

MCDenny,

 

I'm mitivac user too and while never tried tool with bleed nipple closed, I assume you're seeing air bubbles from mitivac's plastic tube/brass nipple junction. If no brake fluid oozes out when you press hard on brake pedal and pedal is firm, well, I'd say you're good to go.

 

Wooster

[Stan Walker]

Screwing in a bleed valve seals off the passage from the center passage to the fluid side of the bleeder (the brake system). There is no guarantee that it will seal off the center passage to the outside world. It's probably passing air from the outside up the threads and over to the center passage.

 

Nothing to worry about.

 

Stan

[Matts_12GS]

McDenny, did you have to do anything different with the ABS system when you bled the brakes? Mine are do and I don't want to take it to the dealer.

[TKNorCal]

Every time I used a Mity-Vac (wonderful device) on my 96 R1100RT I also saw a stream of bubbles in the line. I assumed that the Mity-Vac was pulling a little air from around the connection where Mity-Vac line fits on bleeder valve. Did the bleeding procedure numerous times and brakes on my bike worked fine.

 

Tandy

[MCDenny]

Matt, After reading a lot, I just drained the fluid out the three bleeder valves on the calipers. I did not remove the calipers or compress the brake pads.

 

The whole process wouldn't have taken a half hour had I not been worried about the bubbles and fooling with the bleeder trying to eliminate them. The $30 Mity vac made it an easy one person job.

[flars]

Almost everytime you use it, you will see bubbles streaming up the hose toward the mityvac. These bubbles are coming from between the top of the bleed screw and the hose, and NOT from the bleed screw.

A good practice to follow is to always circumcise the last quarter inch of the hose before you do any bleeding. This will usually ensure a good tight fit around the nipple.

It only sounds bad...the hose doesn't really feel any pain.

You should also buy new hose periodically to ensure the hose doesn't get too brittle to make a good seal around the bleeder screw.

Also - don't overtighten bleeder screws.....

[No_Twilight]

Everyone's entitled to their own opinion so here's mine. I don't like the mightyvac. The pressure in the brake system should never be a vacuum or you're asking for trouble. Speed bleeders do the same thing but use pressure instead of vacuum. I think they're be best thing since sliced bread. pump fluid through using your master cylinder rather than sucking it through. The most vulnerable joint to the vacuum is where you have to loosen the bleed valve to allow suction. Speed bleeders put a sealant on this joint. Cheers, Jerry

[Ken H.]

I don't like the mightyvac. The pressure in the brake system should never be a vacuum

I'm with you. All though many people use them successfully, they just seem counterintuitive to me. Pushing seems more full proof than pulling for some reason in my head.

[Stan Walker]

Well, that's just not the way it works. Suction doesn't pull, it just reduces the pressure on the fluid at the caliper end and allows atmospheric pressure to push it through from the master cylinder.

 

Stan

[Big_Gray]

Of course you are absolutely correct Stan...and atmospheric pressure will push the air anywhere else in your brake system that is not air tight and at reduced pressure.

[Ken H.]

Well it's a bit of an academic discussion (but BMW riders are famous for that right? ) but to certain extent I think Big_Gray has a point. The two method are not apples to apples. In one, using the master cylinder to create positive (relative to atmosphere) pressure in the system will force any undesirable "stuff", including air bubbles, out. The other using a negative pressure in the system (created by the vac) will, at least at the theoretical level, draw air, contaminates, etc. inward.

 

I've always thought the Mity-Vacs were a great tool for speed, but not necessarily the best way to do a bleed. Plus I think they can be hard on master cylinders because you are pulling the fluid out of the reservoir past the piston, instead of the piston pushing it down the cylinder's chamber as it is designed to do.

[No_Twilight]

My only concern is pressure relative to atmosphere. That's an important reference and in this case, keeping it positive ensures bubbles don't enter through joints. The point being made is that "sucking" isn't really sucking but it is just lowering the downstream pressure so that the upstream pressure can push. I agree with that. But trust me on the speed bleeders. --Jerry

[Stan Walker]

keeping it positive ensures bubbles don't enter through joints

 

If your joints leak air..... your joints will most likely leak brake fluid too. I suggest you fix them.

 

Stan

[No_Twilight]

I think I mentioned in my original post that the most likely place for air to leak is the joint you have to loosen to do the bleeding. Clearly Mighty vac can work, many are using it. I just think there is less margin for error and more liklihood of problems. Remember I started out that this is just my opinion. Cheers, Jerry

[Stan Walker]

Jerry

 

The problem with using the master cylinder to "push" the brake fluid is that full strokes often move the piston into areas of the bore that it doesn't normally travel. These areas are often cruddy and pitted and can cause damage to the rubber seal on the piston.

 

Stan

[Ken H.]

Jerry

 

The problem with using the master cylinder to "push" the brake fluid is that full strokes often move the piston into areas of the bore that it doesn't normally travel. These areas are often cruddy and pitted and can cause damage to the rubber seal on the piston.

 

Stan

Well that's a good point too, and one we often forget. Not to bottom out or 'over travel' the master cylinder's stroke when bleeding. I've seen a couple of master cyl. that started leaking sometime after a bleed, and this is the likely reason.

[Wooster]

My chief beef w/speed bleeders is that with my "old fashioned" oilhead hydraulic system, it takes just about forever to cycle 10 or 12 ounces of fresh fluid using bike's master cylinder as pusher; mitivac works so quickly that I ask a bud to assist, pouring fluid into master's reservoir. OTOH, the more recent servo system relies on bike's pump to move fluid thru system, negating my beef. As wise man says, "just my opinion".

 

Wooster wanting to be wise man

[wolcott]

I had a new front master cylinder put on my bike by the local bmw shop. They underestimated the time 'cos of heated handlegrips, so they told me I could bleed them myself or the shop could do it for extra money. The mechanic said there was no air in system, but I needed to get rid of old fluid. I decided to bleed it myself with the mity-vac. Read a couple of tips on doing it. Started on rear caliper. Pumped it up to 10 psi and watched gauge. It dropped to zero about 1 psi per second. Would not hold good vacuum around bleeder nipple. Front caliper-left hand nipple held good vacuum- pumped up to 10 psi and it stayed there. Went to r.h. front nipple and pumped it up to 10 psi-it dropped even quicker than the rear. I did not crack the bleeders on any of the calipers, I just did this test with the mity-vac on the brake bleeder nipples to see if they would hold vacuum. I trimmed 1/4" of hose off the end, and the hose was in good shape with a relatively thick wall. I figured if the nipple/hose interface would not hold vacuum, what was the sense in trying to bleed the brakes this way- one short article I read said to hook up mity-vac to brake bleeder nipple and pump up to 10 psi and see if it holds- if it doesn't, you can't proceed. I got ticked off and ordered speed bleeders. I posted a question about how far I should bring back the brake levers while bleeding, as I had a new master cylinder. A few people responded. One person said it wouldn't hurt to be careful of my lever strokes, the other said to use full strokes. When I bleed the brakes, I think I was careful not to use full strokes, but maybe I did a few times. A couple of days after bleeding, I noticed a drop of fluid hanging off the bottom of the master cylinder. I took off little plastic cover, and there was a little fluid in there too. Overall, maybe 1/4 of an oz. Question; 1) Do you think I overfilled master cylinder (siphoned some out with baster) or did I inadvertently use some full strokes while bleeding and damaged master cylinder seal? Since I siphoned some out, I see no more drips from master cylinder. 2) I know when bike is on center stand ('98 r1100rt) and handlebars level that you should see glass sight in master cylinder full. When on side stand, shouldn't you see the sight glass like 3/4 full, with some space on top? I was having a hard time figuring out what is the proper level to fill to in the front master cylinder without overfilling. All advice appreciated.

[Wooster]

Hi Wolcott,

 

Your post raised questions/issues with feeble minded me (have cold four days, a bit messed-up), namely: 1) if new master cylinder, doesn't that require "bench bleed" (I've never replaced master cylinder yet heard others assert bench bleed needed; perhaps shop mechanic did this allready, what with "no air in system" statement, yet I don't know); 2) the "little plastic cover" you refer to, that isn't the four screw metal top to fluid reservoir, is it; 3) I agree with your assumption that full fluid level means no visible gap in site glass with bars (or maybe more accurately said, master cylinder) level, thus, tilting bike to left (side stand) should produce a bit of a gap (too bad bmw doesn't reference a ledge or other mark inside fluid reservoir as as "full"; my honda XR has such a mark); 4) my mitivac is "econo" model, plastic w/o preasure gauge; when refreshing fluid, I generally continue pumping mitivac while bleed nipple is open and only stop pumping when I close nipple; your observed preasure drop (great Toots and the Maytals song btw) may not be critical and is likely source of harmless small air bubbles seen when drawing old fluid out (old fluid w/o air that is); fwiw, my econo mitivac came with a smaller internal diameter adapter tube (short, with union to connect to long tube) which gives me a pretty tight fit; 5) lastly (I'm confusing me), I don't think you should worry about too full strokes on front brake lever as potential problem (piston encounters crud build-up) isn't likely given new master cylinder. I'm losing my train of thought yet recall bad effects of hydraulic fluid on paint so any leakage could leave unsitely mark on fairing; I've never accessed bleed nipples on abs box when annually refreshing fluid yet replacing master cylinder would make me think about doing that.

Absolutely lastly, if others don't weigh-in, consider sending pm to moderater Mitch (can't recall his name dejour, something like snoady or roady).

 

Adios,

 

Wooster

[wolcott]

Wooster: The mechanic at bmw who replaced my master cylinder (front) has obviously got some trick he uses when replacing master cylinder so that the whole system doesn't get fill of air. Maybe they do bench bleed them first. I think you're probably right that I did not hurt new master cylinder by giving it any full strokes (really can't remember!) as it had no crud built up in there yet. Therefore, it must have been overfilled. I siphoned some out and have not had any fluid leaks since then. Yes, it would be nice if they would put a reference line on the inside of the front master cylinder. Bike is apart now, installing faulty LH heated grip and awaiting arrival of odyssey battery to clear up ABS faults. Regards, Michael

[Ken H.]

Do you think I overfilled master cylinder (siphoned some out with baster) or did I inadvertently use some full strokes while bleeding and damaged master cylinder seal? Since I siphoned some out, I see no more drips from master cylinder.

If you damaged a piston seal in the maser cylinder it would probably continue to leak. And see as yours isn't, I'm guessing it was spillage. It doesn't take much to look like a lot.

[upflying]

Sorry if this is a slight hi-jak. When using the Mity-Vac to bleed the brake calipers, does this also bleed the ABS unit under the tank?

[Wooster]

Well Bob,

 

I think so; that is, by drawing out sufficient quantities of hydraulic fluid from front and rear caliper bleed nipples, fluid in abs unit joins the stream and cycles thru. OTOH, I've never actually attached my twenty doller mitivac tool to the abs bleed nipples and recognize that there may be some alternate opinion vis a vis annual fluid change w/o directly drawing fluid from abs nipples. Service manual recommends this extra effort (direct bleeding of abs unit) yet I've not done this in the eight years I've enjoyed my oilhead RT.

 

Here's hoping my brakes don't come to grief; so far, A-OK.

 

Jon (Wooster's best friend)

 

btw, heard about the tv antennas meeting on roof top, falling in love and getting married ? the ceremony wasn't much but the reception was excellent; more relevant to bleed nipples is the dyslexic man walking into a bra.