Hello everyone. I've searched and can't find exactly what I'm looking for. 2008 coupe w/Z51. I removed the calipers to have them powdercoated by Mike (corvette pilot), great job by the way. Reinstalled them, and yes they are on the correct sides, all bleeder screws are on top. I also marked them before I shipped them to him. Tried pressure bleeding at 30psi like the shop manual calls for. The fronts flow pretty well, but the rears don't get a full steady stream of fluid that will fill the clear hose attatched to the bleeder screw like the fronts. It barely trickles out. I figured it should come gushing out at that pressure but it doesn't. Tried vacuum bleeding the right rear and with the bleeder screw open I actually created a vacuum, and the fluid came out very erratically. Put it all back together and the pedal was soooo soft. Got a helper, and manually bled the rears. Still, only got a small amount of fluid out each time before the pedal drops to the floor when the bleeder is opened. The pedal was a little better, but nothing like it's supposed to be. I plugged the brake lines when the calipers were off, yet some fluid still escaped, but not much. The rear of the master cylinder never ran dry, although I can't be certain about the front since you can't see it looking in the cap. I've bled many brakes on many cars before and never had problems like this. My questions are, even if the brake lines were to completely empty themselves while the calipers were off, which didn't happen anyway but, shouldn't I still be able to bleed them normally? Is it possible that the front of the master cylinder went dry, or are the baffles and slots designed in such a way that if the rear half had fluid the front would be at the same level? I've had master cylinders run dry before on other cars when you either are bleeding and not paying attention to the level or when I had a brake hose break and that did not require bench bleeding it, only new master cylinders when the lines attatched to it were disconnected. Or, on this car would the master cylinder running dry, if it happened, be introducing air above the abs components? If the abs is not engaged, wouldn't fluid go from the master cylinder past the open abs valves to the calipers like any other car, therefore normal bleeding should be possible? Does the ignition have to be on for some reason when bleeding to enengize the abs valves and allow normal fluid flow past the abs components? Still can't figure out why I got a vacuum on the right rear instead of steady fluid. Do you think I need to bench bleed the master cylinder? Do you think I need to get then right scan tool and bleed the abs system? Please help. Thanks.

 

The abs can't be opened for bleeding with out a Tech2 or the like, as far as I know.

 

The rears always seems to bleed slower than the fronts.. as you say.. it trickles out.
even using my mty-vac, I still hardly get fluid out of the rears. Not sure why you are having these issues... I doubt you need to bench bleed the master. You don't need the scan tool for the ABS to bleed the system.
It sounds like you still have some air trapped. I use a pressure bleeder with good results. Did you follow the recommended pattern? I think it starts pass rear, drivers front, etc... not exactly sure..
Try the procedure again with the proper sequence... sometimes the air gets trapped in the brake line distribution block which is why they have a rotation sequence.. good luck

 

keep bleeding in the rear. There is a sequence, i dont remember off the top of my head what it is. I had a very good stream of fluid coming out when i did mine. I also just had someone pumping the pedal. Also, i did it with the engine running. Hope this helps

 

if it wasnt so crappy outside i,d take a drive and see if i could help ya out

 

I had the same issue and I found that by continuously pumping the brake pedal it eventually worked the air out of the system.

 

Start the car, and have someone pump the brakes while you work the bleeder screw manually.

 

I noticed you said the bleeders are all on top. I just did mine and I think I have them on the bottom on the front. So I guess I need to take them off and switch sides?

 

Yep...you definitely have them upside down.

Usually you have a very hard time bleeding and getting a firm pedal when they are upside down like that.

Bob

 

Thanks I was wondering what to do today. Now I know.

 

You should bleed with the engine shut down and ignition OFF.

The Service Manual says that after after raising the car and removing the wheels, you should pump the brake pedal 3 to 5 times (or until there is an increase in pressure) to allow the brake booster pressure in the system to be relieved.

The master cylinder has a baffle in the middle to divide it into a front and rear reservoir - you add fluid to the rear and when it gets to a high enough level it flows over the baffle into the front reservoir.

For safety, the brake system is divided into 2 circuits - the RR/LF, and the LR/RF. In the event of a leak or damage to one of the circuits, the other one should be able to stop the car.

I think the front reservoir in the master cylinder supplies fluid to the RR/LF wheels, and the rear reservoir is for the LR/RF.

The Service Manual says when bleeding to bleed in the order RR, LF, LR, RF.

So....all that still doesn't give any good insight into the OP's issue of not being able to get a good fluid flow from the rears when bleeding.

The only thing I can think of is that perhaps there is something restricting fluid flow through the rear calipers. I have a Motive Power Bleeder, and I only need 10 to 12 psi to get a good fluid flow when I open the bleed valves. You may have to remove the rear calipers and make sure the fluid path isn't clogged by debris.

Good luck!

Bob

 

Air goes up, not down, so bleeders go on top.

What I saw a Harley mech do once, was to put a vacuum hose connection into an extra MC cover and use the mite-vac to bleed from the pads to the MC. Worked great. On mc's I get a large syringe and pump the fluid from the caliper to the mc. Works great.... for those that have Harley's and do their own maintence. Not sure this would work on an ABS sytem, let alone a brake system as large as a car. You'd need a very large source to pump the fluid thru that much hosing.
John

 

Ok, I've been doing some more research and see that I'm not the only one to have trouble getting good fluid flow from the rear. The car has less than 400 miles on it so I don't think debris is an issue. I read the car has no proportioning valve and that the abs valves limit fluid pressure to the rear and allow full pressure to the front under normal driving. If that's the case, it makes sense why it's so hard to get fluid flow from the rears. I know the manual says engine off and remove booster pressure by pumping the pedal first until it gets harder but would turning the ignition on open the valves and allow easier bleeding in the rear? If so, from what I see there is no traditional "on" position, with the engine off, just Acc. Thanks for all the help, keep the ideas coming please.

 

i did mine with the engine running. my pedal feels great.

 

Keep your foot off the brake and continuously hold the button for 5 secs or more and you will get "ignition on."

Edit: I should have been more clear in saying to push on the bottom of the button. But still keep your foot off the brake just in case you push more on the top of the button to keep from starting the car.

 

If you are still having trouble after trying to bleed all the calipers again, you might try cracking the fittings at the master cylinder and making sure there is no air trapped there.
Push on the pedal just like you were bleeding the calipers, crack the brake line fitting at the master cylinder and make sure no air bubbles are coming out. If you did let one side of the master cylinder go dry, and you do not get it bled, you will never get all the air out of the system.
I agree that the ABS valves should be normally closed so you should be able to manually bleed the system without actuating the ABS system.

 

To get to the ignition on, engine off position, press the bottom of the start button and hold (5-10 sec). The car will first show Accessory Mode then will go ignition on mode.

 

Because you only removed your calipers, I still think you should just get a helper to start the car, pump the brakes, and bleed manually. I bet you will hit solid fluid real quick, and just tighten her up. Keep resevoir full.

 

I have flushed/bled my brakes many times before and after track events. I have never had the engine running, and I've never had any problem getting a good flow of fluid out of the bleed valves. The manual does not make any big deal of making sure the ignition is off for a normal bleed. Whenever I do a flush/bleed, I shut down the car and by the time I've got it up and the wheels off has been at least 10 minutes or so since I shut the ignition off.

When bleeding the ABS with a Tech 2, the Service Manual tells you to turn the ignition ON with the engine OFF.

However, if you are servicing the EBCM (Electronic Brake Control Module) or the BPMV (Brake Pressure Modulator Valve), the Service Manual says:

IMPORTANT:

• The ignition switch must be placed on OFF mode ONLY, before disconnecting the EBCM. If the ignition is accidently placed into ignition ON engine OFF mode, the control modules will be activted and dixonnecting the modjule or negative battery cable will cause various DTCs to be set.
  
  
• After the ignition switch is placed into OFF mode, a waiting period of 2 minutes must be observed to allow the high-speed LAN control modules to deactivate before disconnecting or connecting the module or negative battery cable. If the waiting period is not observed, one or more of the control modules may still be active; tihis will cause various DTCs to be set.

As I said above, the manual doesn't make any mention of ignition ON or OFF or any waiting time before doing a normal bleed, but the above does indicate that for a period of time after shutdown some of the valving in the EBCM/BPMV may still be cycling. That could cause a problem when trying to bleed the system. By the time I'm ready to bleed, the car has been shutdown for several minutes and all the valves should have been where they need to go after ignition shutdown.

Bob

 

You're correct that there is no mechanicl proportioning valve, but rather a Brake Pressure Balance Control System that regulates pressure to the brake circuits.

Although there is no proportioning valve, there is DRP (Dynamic Rear Proportioning), which is a function of the ABS modulator. The Service Manual says:


See my previous post above with info about waiting at least 2 minutes before servicing the EBCM/BPMV. If anything, I'd recommend making sure the electrical system was shutdown for a couple minutes to allow those valves to go where they're supposed to go on shutdown, which might actually allow the rear pressure regulator valves to go full open to allow a better flow of fluid during bleeding.

Good luck!

Bob