brake bleeding

[cmcfaul]

I replaced all fluid and installed new pads (Hawk 5.0). Used a vacuum bleeder. Air gets in through the threads of of the bleeder screw. Should not matter but bias has shifted to the front so want to bleed the rears again using a different approach. How do you seal the bleeder screw so to not let air in.

Thanks,

Chris
89 Carrera

[Bill Douglas]

I used speed bleeders.

[cmcfaul]

just checked, Speed Bleeder does not support an 89 Carrera. I imagine any 8 mm bleeder nipple would work.

Chris
89 Carrera

[HarryD]

I have always had issues of minor bubbles with vacuum bleeding. Went to pressure bleeding with a Motive unit and have never looked back.

[cmcfaul]

even with pressure bleeding, once the bleeder screw is opened air will get in through the threads, yes?

Chris

[aston@ultrasw.c]

you have discovered the Achilles heel of vac bleeding

some people have suggested wrapping teflon tape around the valve

however i agree that a pressure bleeder is the way to go

[aston@ultrasw.c]

Originally Posted by cmcfaul

even with pressure bleeding, once the bleeder screw is opened air will get in through the threads, yes?

Chris

not in my experience, some fluid may weep out but air does not get in

[Quicksilver]

Vacuum bleeding can pull air past seals. The seals are designed to contain pressure not vacuum.

That said, unless there is a huge quantity of air in the system it won't change the bias unless the pistons in the master cylinder are hitting each other. The master cylinder produces the same pressure out of both ports. Even if the quantity of fluid and/or air in the front and rear systems is different they will both have the same pressure.

Either there is a change in the friction level in the front and rear brakes, there is a change in the effort required to actuate the calipers in one end. Or there is a huge quantity of air in the systems so the primary piston is hitting the secondary piston (which would require large pedal movement).

[cmcfaul]

well front pads are definitely working harder than the rears now. Went for a hard drive with constant threshold braking. The fronts were smoking hot the rears not a all. Pedal feels a little spongy. So going to bleed the rear brakes the traditional way (two person method) and see if it helps. Front locks up way before the rears now. Has to be a bleeding issue.

Chris
89 Carrera

[HarryD]

Originally Posted by cmcfaul

well front pads are definitely working harder than the rears now. Went for a hard drive with constant threshold braking. The fronts were smoking hot the rears not a all. Pedal feels a little spongy. So going to bleed the rear brakes the traditional way (two person method) and see if it helps. Front locks up way before the rears now. Has to be a bleeding issue.

Chris
89 Carrera

If you have a firm pedal, there is little if any air in the lines.

Verify the fluid flows easily when your bleed, otherwise you have a restriction in the brake hoses.

Or you could have frozen pistons.

[stownsen914]

Originally Posted by cmcfaul

even with pressure bleeding, once the bleeder screw is opened air will get in through the threads, yes?

Shouldn't, since with a pressure bleeding system (or bleeding the old fashioned way) there isn't normally a vacuum acting on the bleeder. I suppose some brake fluid could seep out the threads, but this won't introduce air into the system.

Re: speed bleeders, I tried them once and killed almost an entire day at the track chasing air in the system. Luckily I'd brought my old bleeders - put them back in, no more issues.

[Mahler9th]

How do you know for sure that bias has shifted to the front?

I am pretty sure that all street cars have extra front bias by design. Lots of ways to move it back, even for street cars. A common method for some PAG models back in the day was to install a different brake proportioning valve from the various stock factory units. I still remember Kelly Moss Racing suggesting I switch from a stock 951S valve to a 965 valve, which cost about $40 at the time. Worked well for my application.

I have also experimented with different pad compounds front and rear. Lots of ways to monkey around with bias.

I have used a Mityvac for street car and race car bleeding for 30 or so years. My cars and friends cars. Many types of calipers. No issues.

I have never tried to figure out a way to stop air from being sucked in past the bleed screw threads.

I simply close the bleed screws while there is still strong vacuum. I have never drawn conclusions from "looking for bubbles in the tubing." Those bubbles can come from air sucking past threads in the various types of bleed screws.

I have two Mityvac units, one in my shop and one in my race car trailer.

I also use a Motiv bleeder at times. No particular reason- just a change of pace. The owners of Motiv are friends and they have a great family of products.

I have dealt with many brake system challenges in Porsches through the years-- the weirdest one was a leaky MC seal at the booster in a 951s. Fluid got in the booster and caused all kinds of problems. It presented in Friday afternoon traffic when I was on my way to Laguna Seca for a PCA driving weekend. I'd get about 20 minutes of driving in traffic done, and then the front brakes would lock on and I'd have to pull over. Finally ID'd the issue Saturday afternoon, and missed the track driving weekend. But I learned.

[jlex]

I've also used both the Motive and Mityvac systems and prefer the Mityvac. Seems to do the job more quickly and has a quart bottle that can be mounted on the reservoir to make sure you don't run out of fluid. A few bubbles being sucked in at the valve is no big deal. If the fluid is flowing clean, it's pretty obvious the job's done.

[chrismorse]

Progress is slow on restomoding my dad's 74. Recently, new PMB calipers, discs, (trued), bearings, SS hoses and new MC.
Mightyvac bleeding was slow, I suspected bubbles coming from a slightly loose tubing fit to the bleed screw - I used a tie wrap to snug up the tube to bleeder connection - still had a lot of bubbles, so tried teflon tape on the bleeder screw threads, (not on the taper seating area - don't want bits of teflon drifting about in the system). This combination made the mighty vac system bleed fairly quickly.

I have heard of potential air being sucked past the piston seals and wondered if that was a problem with old seals, or too much vacuum, so i do not really pull a lot of vacuum to do the bleed and this has worked well on many brake jobs.

While i was a service manager/service writer at a GM/Subaru/FIAT dealership inthe 70's, i was the designated pedal pusher for brake bleeding. During that 10 year stint, i noticed a significant, (not huge), number of master cylinder failures, - sinking pedal, inability to bleed, immediately on concluding the usual pedal UP/ down bleeding dance. My conclusion was that the MC seal was failing because of the full, long stroke of the MC piston going over rusty trash build-up in the bore, at the end of the normal brake MC piston travel, cutting the seal. I credit this trash build up to brake jobs being done very infrequently on the Detroit Iron and insufficient annual/bianual brake flushing.

I have yet to try pressure bleeding, (air only) and have been happy with vacuum bleeding, once the kinks were worked out.

chris

[cmcfaul]

bias was fine. I have to believe Porsche designed the brakes with perfect bias with matching compound. Will put some teflon tape on the bleed screws and bleed the rears some more. Rear pads were wearing 2x faster then the fronts. Bet that is reversed now.

Chris
89 carrera

[Cory M]

I've used the hell out of the Motive bleeder I bought for $40 15 years ago. Probably one of the best tool purchases I've made. Other than me being a dummy and forgetting to clamp the overflow line it's been trouble free and works great.

[Jeff Higgins]

I have lost track of how many friends' and acquaintances brake systems I have miraculously "fixed" by simply bleeding them the old fashioned way - one guy pumping and holding the pedal, the other cracking the bleeder valve on the caliper. You simply cannot introduce air into the system if you do this properly. Pretty much every other way, using some fancy pressure or vacuum bleeder, will.

[cmcfaul]

Read the reviews on the Motive pressure bleeder. They are very positive. May get it just to have for next time. For now, gonna make the girl friend pump (the brake) while I (loosen and tighten the) screw

Chris

[Mike Andrew]

Make sure she understands not to push the pedal to the floor. Or, slide a block of wood under it to fail safe this.

[cmcfaul]

What happens if you push the brake to the floor?

Chris
89 Carrera