How do I trap air in to the HCU?

I am doing some brake line jobs. What I have been reading is that one should be careful not to trap air inside the HCU. The tool was quite expensive...

So how do I trap the air into the HCU?
Is it backwards from the HCU to the the brakes or only if I run out of fluid and pump air into the HCU from the master cylinder?


Regards


[EDIT: I see now that this should be posted in the stock forum and not in the modified...]

Two ways:

1. You let the fluid level in the master cylinder run out and get to the level of the HCU (HCU is between master cylinder and wheel cylinders)

2. You disconnect a wheel cylinder and lose all fluid in the line up to the HCU.


If you get air in the HCU, you need a tool that will cycle the solenoids in the HCU to get the air out. The factory tool is expensive, but there are aftermarket scanners which can do this. I have a Diagtek scanner which can do this, but I haven't tried yet.

What is HCU?

Jason94sport said:

What is HCU?

Click to expand...

Hydraulic Control Unit. It's for ABS.

dogfriend said:

If you get air in the HCU, you need a tool that will cycle the solenoids in the HCU to get the air out. The factory tool is expensive, but there are aftermarket scanners which can do this. I have a Diagtek scanner which can do this, but I haven't tried yet.

Click to expand...

Are there scan tools for OBD-I computers that will do this? You have a 97 Explorer (OBD-II), so you have plenty of scanners out there that could kick on the solenoids...but are they any for OBD-I? Will your Diagtek work on an OBD-I?

Rhett said:

Are there scan tools for OBD-I computers that will do this? You have a 97 Explorer (OBD-II), so you have plenty of scanners out there that could kick on the solenoids...but are they any for OBD-I? Will your Diagtek work on an OBD-I?

Click to expand...

Glacier found a tool that is for ABS only. I don't know if it works for OBD-I.

Dead Link Removed

The DiagTek is for OBD-II only.

Ahh, I always called it the ABS unit. I was told the only way to get air in that is if you pull the master cylinder, or pump the brakes with zero fluid in them.

Thanks for the replies... So ... a little bit confused then.... Is it then impossible to change breaklines from the HCU down to the front wheel without getting air into the HCU?
I mean, if I remove the brakeline from the HCU the air can directly get into it. Or can I do it if I am just fast enough?

Another idea... If I create a small continious overpressure at the filling cap and keep the fluid pouring a little bit out from the HCU and in the meantime I just switch the brakelines quickly?
Would that be a way to go and prevent my issues..?

Peter Westling said:

Thanks for the replies... So ... a little bit confused then.... Is it then impossible to change breaklines from the HCU down to the front wheel without getting air into the HCU?
I mean, if I remove the brakeline from the HCU the air can directly get into it. Or can I do it if I am just fast enough?

Click to expand...

Now I see the real question you are asking... Yes you can remove the brake line fittings @ the HCU and not have problems with air in the system. I have replaced the entire rear brake line from HCU to rear axle as well as the driver's front hard line (HCU to caliper). Both jobs were on different days and I never had a problem with Air getting into the HCU... Before removing a line I would fill the brake fluid reservoir above max to prevent the fluid level from getting too low. I would then remove the fitting from the HCU and insert a tappered rubber plug into the HCU hole to prevent the fluid from leaking out... I checked the reservoir several time during the repaire and a top off if necessary.

NOTE: The Rear hard line from the HCU back was a std 3/16 line and fittings while the front lines were a combo metric/std (metric @ HCU, Std at coil bucket). See Brake line size thread for related info.

There you go! I am very pleased with the answer. It might be my poor english here.. Kind of hard sometimes when discussing in specific "car-english" terms, we did not go through that in school..

Anyway, Derocha, I feel confident to proceed. I actually called up some Ford specialized mechanics here in Swe but noone had or heard about the T90P-50-ALA tool, They replied that the NGS computer MIGHT do the job though.