Brake bleeding

[kenai]

Wrapping up knuckle rebuild and new F & R brakes. Bled the rear brakes just fine. Went to bleed the front and I cannot get any fluid to come out of the bleeder screw and noticed that the pistons were not moving. Suspected blocked line so I took them apart and blew them all out with compressed air, no blockage.

Went to bleed again and still no fluid coming out of the front bleeder screws and no movement on the pistons. WTH? Turned the vehicle to the on position just in case the ABS requires power, still nothing.

So, I disconnected the lines at the calipers to see if they are bad and no fluid will come out when brakes are depressed.

My buddy pushing the brakes said that when I cracked the bleeder he felt no change in the brake pedal.

Disconnected the lines at the ABS module and fluid came out when pedal was depressed. I am lost. Moving in a few days and stressed about getting this back on the road. Help is appreciated!

 

[kilmarkid]

Had a similar problem when I replaced the front calipers. Went to bleed the rears and one would not drip at all. Took a nipple from one of my old front calipers and threw it in the rear. Worked like a charm but sounds like your case may be different. Remove the nipplenand see if it drips.

 

[kenai]

I wish that was the case. I have blown out all the nipples or bleeder screws. Is there a combination valve or something that would prevent the fronts from getting fluid?

 

[kenai]

I am beginning to think maybe the master cylinder is bad. My brake pedal is slow to return up. I just don't really understand the brake system on the 80. Two lines from the Master Cylinder. One goes to the ABS module and the other goes to a T fitting with one end going to the ABS and the other to the rear LSPV.

Then There are 3 lines out of the ABS Unit. 2 to the front brakes and one to the rear LSPV.

What the Heck would cause just the front brakes to not operate?

 

[Hoosier Daddy]

I agree, master cylinder is the likely culprit

MC has two circuits - one for front, one for rear. Both go to the ABS module, where the front is split into two front channels and the rear line continues as a single channel to the rear brakes. That rear line passes through the LSPV, which controls pressure to the rear to compensate for vehicle load.

The LSPV also has a line going to it from the front brake circuit (from that T you saw earlier). That line from the front circuit does not apply braking pressure, but it controls a bypass valve in the LSPV. In the event of front brake failure (like you probably have), the system no longer sends pressure to the LSPV bypass valve and the valve opens. This allows full braking pressure to the rear brakes to compensate for the lack of front brakes.

So if your MC has a bad seal in the front circuit you will not build any pressure in the front brakes or at the LSPV bypass valve. Your rear brakes will be doing all the work.

Did you check for fluid pressure before or after the ABS module?

You can detach the line from the MC that leads to that T (this is the front circuit) and see if you get much fluid pressure from that hole in the MC with pressing the brake. If there is little or no pressure at that port, you know the problem is your MC.

Makes sense?

Be prepared to bleed this thing over and over and over after you replace the MC. Do everything you can to minimize the amount of air in the system, and whatever you do, absolutely do not let the MC run dry when bleeding it.

If you want to learn everything about Toyota brakes, go to the following link:
http://www.autoshop101.com/forms/brake08.pdf

You can change the digits on the pdf file number (01-13) to see the whole series. Set aside about 5 hours to read and understand it all.

 

[kenai]

Hoosier Daddy, your my daddy. Thanks a million! I did check for pressure after the ABS module. I was nervous about shooting brake fluid sky high so my friend slowly depressed the pedal until I saw fluid come out. Not sure if that means it was working effectively or not.

I do get a bump in the pedal when I push it down. Seems like it goes halfway then then I feel a bump or slight click as I continue pushing it to the floor.

I think at this point I will be ordering a MC. Nobody has one in my city so it looks like I will be parking the cruiser at my neighbors for the move, then coming back to fix it and get it to our new place.

Recommendations? Reman or new Asin?

 

[Hoosier Daddy]

Hoosier Daddy, your my daddy. Thanks a million! I did check for pressure after the ABS module. I was nervous about shooting brake fluid sky high so my friend slowly depressed the pedal until I saw fluid come out. Not sure if that means it was working effectively or not.

Yeah, hard to know what to make of that. Better to avoid opening the ports directly on the ABS module unless you have to. Do everything you can to keep air out of that thing.

Assuming you are getting flow past the ABS module, the next place I would check for fluid pressure would be right behind the front tires where the individual front hard lines coming from the ABS module meet the rubber lines that drop down to the axle. (at the top of the rubber lines) Don't worry about spraying fluid, as the pressure should be like normally bleeding brakes. If no fluid pressure here when depressing the pedal, then I would further investigate the MC, as described below. If you do get fluid pressure here at the top of the rubber lines, your front rubber lines could have internal damage preventing fluid passage. Look for crimps or kinks in your hard lines too. You never know.

To check the MC, I would disconnect that front brake circuit line from the MC, then put your finger over the port. Have a lot of paper towels handy and some water to rinse off any brake fluid that lands on your paint. Pressing and holding the brake should cause pressure behind your finger. Removing your finger with the brake held down will let you know how much pressure there is. Slowly dripping means nothing - you want pressure. Cover the port with your finger again before your friend releases the brake pedal, to prevent sucking air into the MC. Be very careful not to booger up or cross-thread the fittings during the process of removal and assembly of that brake line.

If your MC is actually moving strong amounts of fluid and you have no fluid pressure at the top of your rubber lines, I would start thinking about the ABS module - but let's not go there yet.

I do get a bump in the pedal when I push it down. Seems like it goes halfway then then I feel a bump or slight click as I continue pushing it to the floor.

Not sure if that's diagnostic of anything in particular.

I think at this point I will be ordering a MC. Nobody has one in my city so it looks like I will be parking the cruiser at my neighbors for the move, then coming back to fix it and get it to our new place.

Recommendations? Reman or new Asin?

YMMV, but i say replacing the MC is such a PITA, I only want to do it once. Buy the best part you can afford.

 

[Hoosier Daddy]

BTW, what brake parts did you replace, what did you blow out with compressed air, and how were the brakes before this project?

Also, I looked at your past posts and see you have a lot of mechanical experience. Apologies if I've been telling you things you already know.

 

[kenai]

Thanks, I'll check for pressure right after the MC. I have replaced the rubber lines and have blown out both the hard lines, soft lines, and bleeder valves. I know for sure that I don't have any issues with a blockage in the lines. I do not get fluid coming out of the lines, even when disconnected from the Caliper. Correction, I do get a little bit on the driver side but not much.

Calipers are original. I was having a known brake issues before the knuckle rebuild and new pads/rotors. Basically since I bought it. Originally, the abs would activate when I came to a stop. I figured it was due to my super sloppy overdue need for a knuckle rebuild and the sensors were whack. So I pulled the fuse for the time being to keep the ABS from activating. Fuse was replaced after knuckle rebuild/brake job.

Also, the Cruiser would pull hard left under heavy braking. Figured this was due to a stuck caliper or rusty caliper pins on the passenger side. Both calipers checked out great when cleaned up. It really is just a fluid movement issue.

Is it possible for the ABS unit to not allow fluid movement from the MC to the calipers?

I think I already dorked up and got air into the ABS module. Once I get fluid moving again it should be a joy to get bled.

 

[Hoosier Daddy]

Technically it is possible for the ABS to block flow from MC to caliper, since that's partly how ABS works, but it seems unlikely. The 80's ABS system uses 3 solenoids (2 front, 1 rear), so you would likely need to have 2 out of 3 solenoids malfunctioning (stuck "on") to cause no flow to both sides in the front. Anything is possible I guess, especially with the odd behavior it was exhibiting prior to this. Might try pulling that fuse again and seeing if that changes anything.

 

[kenai]

Ok, back from the move. Still got a ton to do but I looked at the 80 for a bit this afternoon. I disconnected the line at the MC and fluid seemed to shoot out just fine. Since its been sitting a while I decided to bleed the system again. Started rear right and got out a few bubbles, then rear left and a few more. Moved to the front and again no movement on the front right and a very small amount on the front left. Then to the rear level proportioning valve and very little movement also.

So, if I have fluid coming out of the MC (after the T) but virtually non at the proportioning valve how could that be?

I am really wondering if the ABS is stuck.

 

[kenai]

disconnected the line from the MC and checked for fluid movement before the "T" and the fluid shot out super hard. So, I think this means the MC is good. But, is it possible that it is shooting fluid out because there is no load? Maybe when I connect everything back up it doesn't build enough pressure to push the fluid, although that doesn't make sense because when I open the bleeders it should be able to push through. I am at a loss.

I am thinking that I have some massive air in the lines and maybe just pumping the pedal can't move it out.

 

[DSRTRDR]

LSPV needs to be bled as well - we did it between every caliper

 

[Chester]

Try this. open a bleeder on one of the rear wheels. pump the brakes a few times and note that the fluid in the reservoir goes down some. Then open a front bleeder and if the resulting drop in the reservoir is similar, good, you just have a pant load of air to be displaced. If, on the other hand, fluid level remains the same, then you probably have a failed master cylinder. try this too. With all bleeders closed, pump it a few times, expecting no fluid drop in the reservoir. That sounds awkward, but you get the jest of it.

 

[Grim Reaper]

The line coming from the front circuit to the LSVP may need to be bled as well. I can't remember. If the bleed nipple on the LSPV is for the front line or rear but if that line has air in it you need to get it out so if all else fails crack open that line at the valve and make sure your getting fluid.

Something that caught my eye is you saying the ABS was activating before your started working on it. What triggers the ABS is a miss match in wheel speed. That's pointing at a ABS issue if your sure their wasn't a wheel wasn't locking. For the sake of bleeding if your running the engine while bleeding the system pull the ABS fuse to make use it's not activating and fighting you.

 

[blunderbuss]

what ever came of your issue? Did you get it fixed?

 

[torquedorque]

Is it fixed?