I have asked this question before and never got an answer that made sense to me. Maybe somebody who really knows for sure will see this. I have a 96. I have seen the posts that showed the screens in the ABS pump clogged and it did not seem like there was much flow. I know that a normal brake system has very little flow when working. My question is: If I do a normal brake flush at each wheel does the new brake fluid flow through these screens that clog in the ABS? Or does the ABS have to be activated to get any flow through the screens? If I knew for sure it did I would do a four wheel flush way more often. Thanks very much, Dan

 

From another forum on a different brand of car.

Not sure the Fords need it though, as in another forum a user referenced a service manual to state, "Here is a portion of the ABS system overview from the factory service manual, it appears that the ABS pump is in a 'normally open' mode, allowing brake fluid to flow thru it during normal operation, so bleeding the brake system will also cause new fluid to be introduced into the ABS pump. The only place where 'old' fluid might be retained in the the Hydraulic Control Unit (HCU) accumulator. However, Ford does not think that this needs special attention when changing out fluid."

Then I find this:
On their own, gravity, manual or power bleeding methods can’t completely flush the volume of fluid that is in the ABS modulator. Inside the modulator, there might be as much as five ounces of dirty fluid after flushing. The only sure way to flush this fluid is to use a scan tool to actuate the valves in the hydraulic control unit. Be aware that the worst fluid is trapped in the ABS valves and passages due to their non-integral design.

Then this:

In my research I discovered that when the car is in park and not running, some ABS systems return the valves to a "home" position so that the system is "open". The earlier mentioned Bosch and Delphi systems do not return the ABS unit valves to "home" without a scan tool. So, I read that those systems will close off and old brake fluid will stay in the unit and not allow new, fresh brake fluid to flow through if you are bleeding the system. Since I did not see a scan tool requirement or procedure mentioned in the shop manual, I am assuming the ABS system on my 2004 automatically puts the ABS unit valves in their home position when the car is in park and turned off, thus allowing the fluid system to be open and all fluid flushed out.

 

This website shows how bad they can get. I sent them a question as to if our units flush out with regular 4 wheel flushing or if a scan tool is needed. Dan

Lies-within (mtechniqueauto.com)

 

It is truly hard to believe but I already received a response to my question. Cut and paste below:

Good morning Dan,


So the C4 Corvette came with many different ABS pumps over the production range. The earlier cars have the Bosch ABS2 units well represented on my website that I routinely rebuild. These went into the cars until the end of 1991, where there were replaced but the Bosch ABS2U/S units. These were for all intents the same unit from a function and internals standpoint, in simplified castings which were essentially extruded aluminum billet. I believe these were the result of steam lined production and tightened production costs as the forging of the original unit were likely more expensive to manufacture and machine.


In 1995 again the C4 saw an update, to the Bosch 5.0 hydraulic modulator which brough the control electronics onboard the pump which did in time create a whole array of new and interesting failures.


More to the point though, manually or power bleeding fluid through the pumps will exchange the fluid sitting in all of the main passageways and in the solenoid valve areas. The volume of flow really isn't sufficient to dislodge schmoo buildup from any surface, the fine mesh filters included.


Using a scan tool to activate the DC motor will help drive fluid out of the storage reservoirs and aid in the exchange of that fluid with fresh. This fluid only sees flow during an ABS event, when the ECU is requesting a pressure increase after it has already dumped inlet pressure back to the master. It is very hard to exchange the fluid under any normal use because there is so little volume of flow, its all static pressure changes that do the work.


Bleeding routines in scan tools usually cycle the pump and each solenoid, but the main intention is to jiggle free air cavitation as tiny bubbles like to cling to surfaces inside the pump which creates a rubbish pedal feel.


The primary failure mode on Bosch 2 and 2U/S units is internal coking blocking flow and or immobilizing moving parts. On the Bosch 5.0 units those generally suffer from control unit failures more than anything else. I have not yet rebuilt a 5.0 but I can tell you the early 5.0 units did share some of limited internal designs with the legacy units and they would be susceptible to the same buildup related failures overtime with enough neglect to routine fluid services.


Feel free to share my response on CorvetteForum with all that may benefit from it.

 

Well with what I have learned in this thread and the fact that I checked my records and it was July of 2019 I last flushed my brake fluid I am fixing to do a complete flush. Dan

 

What are the steps to flush using a scan tool? This scan tool introduction is foreign to me. Sounds like I need to repeat the process agin on my 95.

The 93 is coming up as I rebuild the calipers.

 

I Have no idea. I do not have a scan tool or know anyone else who does. What I have learned is the new fluid in a flush DOES go through the ABS and flush it out. I always thought you only need the scan tool to help bleed is if the ABS system is replaced or let get empty of fluid. Dan

 

The way I understand it - you set up to pressure bleed the brake system. While doing a pressure bleeding - you use the scan tool to activate the ABS valves, so old fluid / air is pushed out of the ABS unit and into the brake lines - where it will be expelled with the old fluid.

 

I was going to post back on the 10th and post#4 seems to suggest what I was going to mention might be the next 'good read' - the 5E2 section of the FSM for '95 & '96 is different than the earlier and I was going to suggest maybe a 'deeper/more thoughtful' read of the 5E2 section for those years might provide newer insights for the '95 & '96. I don't own either year but I have read them in the past.

 

Sounds correct. I am just happy to learn that the new fluid will circulate through the screens and solenoids. I thought I was going to have to go to a big parking lot after an ice storm and keep accelerating and stomping on the brakes for an hour or so to flush it out. Dan

 

I will. Thank you!!! Dan

 

A number or years ago - a friend and I were getting our cars ready for a HPDE - my friend was also prepping his wife's car - as he had convinced her to give it a shot.... . We were doing a brake fluid flush - and my friend went a bit too long before replenishing the reservoir on his wife's GM car. We realized what we did - but too late. We ran more brake fluid thru the system - but the pedal felt like crap. The HPDE was three days away... My friend took the car out and brought it to a gravel parking area, and mashed the brakes a few times - activating the ABS. We re-bled, and the pedal felt better - not anywhere near good enough - but better. We repeated the process, and the pedal felt Much Better, almost normal. One more time, and the pedal felt just fine. So - should the need arise, that process will probably work if you want to flush fluid out of the ABS and don't have a scan tool.

 

Every time you start the car it cycles the abs pump. On my 92 when I first got it the Power booster failed. I replaced it and removed the MC while doing it. I flushed/bled the system. A week later the MC failed. I replaced it and again bled/flushed the system. A year later and my fluid looks brand new. So doing 1 flush, driving it for a week and doing another will definitely get fresh fluid throughout.

 

I wish I really knew what happens for that 2 seconds after you get to 3 mph after starting. Do all three solenoids actually flow brake fluid? Dan

 

Reading post 4, it seems like a good bleed, then go ABS the **** out of the thing, the bleed again.

When bleeding, don't forget to push the caliper pistons all the way back in.....then don't forget to pump them back out before you drive out of the garage!!

 

I don't know for sure. My 92 had 185k miles when I bought it 10/21. It had sat for the last several years as the PO was elderly and passed away. The nephew is who sold me the car and only did a few things (poorly) to get her running. My brake fluid and clutch fluid was really dirty. So with bleeding it, driving a few times and then bleeding it again it did a thorough flush. The methods of slamming on the brakes on gravel to really activate the pump are probably for extreme situations where you have a blockage or something. I went through all of that on my 89 that had issues.

 

Well, I flushed the hell out of it. My wife hates to pump the brakes to flush so I bought 4 Russell speed bleeders. The speed bleeders worked great for the flushing but I could not get them tight enough to seal when done. I was not about to snap them off in the caliper. Sending back for credit. They looked like they were only touching on one side of the tapered area like maybe not machined correctly. I put the originals back in (sealed right up) and talked the wife into doing 10 pumps per wheel so if any air went in putting the old ones back in. Super firm pedal. Thanks for reading, Dan

 

A tip I might pass on to those that experience a spongy pedal from time to time when flushing brakes. I have used the power motive bleeder on several vettes as well as other makes. For some reason, my '96 vette will not bleed in the rear calipers until I get in the car (with motive bleeder still pressurized) and pump the pedal slowly. The brake pedal then is firm and right at the top. I had to do this again just a couple of months ago after changing out the rear calipers for C4 grand sport type (black). My C5 and C7's bleed just fine without having to pump the pedal. I really don't know the reason but I have had to pump the pedal everytime I bleed the C4 (with power motive maintaining 10psi).

 

You can do that same exact thing, but w/o the power motive bleeder, and get the same results, FYI.

 

I would check your rear rubber brakes lines because if you are not able to suck brake fluid out without adding pressure from the brake pedal then you might have an internal collapsed line . Have they every been changed?