What size nut for master cylinder port block off

[Floyd6]

2004 Chevy Tahoe Z71 5.3 - Need to test master cylinder. Can anyone tell me what size flare nut I would use to block the ports after disconnecting the brake lines?

 

[Scottydoggs]

https://www.amazon.com/Dorman-13911-Master-Cylinder-Bleeder/dp/B001SG8ZC0/ref=asc_df_B001SG8ZC0/?tag=hyprod-20&linkCode=df0&hvadid=312064691975&hvpos=&hvnetw=g&hvrand=14060526649202392566&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9003479&hvtargid=pla-435726495565&psc=1

 

[rockola1971]

Test it for pressure output? What problems are you having and what led you to determine you may have a bad master cylinder? They typicall go to the floor when the seals are out in them. Did you have the hydraulic system open? Having a hard time to bleed the system?

 

[Floyd6]

Pedal goes to floor after starting car. Wanted to isolate master cylinder to make sure that is the issue.

 

[exp500]

Cheaper and easier troubleshooting to replace M/C first. If mastervac failed then you will know for sure, and have a good pressure system.

 

[rockola1971]

Pedal goes to floor after starting car. Wanted to isolate master cylinder to make sure that is the issue.

If you cant find any brake fluid leaks then your Master Cylinder is done.

 

[JamesLond]

If you have an endoscope, you can pull the brake booster check valve and look inside for signs of leaks.

Not 100% conclusive, but it's something.

I'd like to know the correct size also.

 

[exp500]

I suppose you could just read it from the replacement specs after you determine which brakes you have via the glove compartment , like this -

More Information for RAYBESTOS MC390806

 

[SnowDrifter]

I suppose you could just read it from the replacement specs after you determine which brakes you have via the glove compartment , like this -

More Information for RAYBESTOS MC390806

Good idea!
I'll also say to address stuff in the thread: This is a valid test for the m/c.
I'll also add for OP: Once you get the lines off, get a m/c bleed kit and bleed the master before you test it. If air is present, it'll skew your tests.

OP: Mind sharing more info on what's going on?
Also to be aware, you'll need to bleed the ABS after this is all said and done. Either command w/ scan tool, or plan on bleeding the whole system 2-3 times. Take the vehicle out on gravel and trigger the ABS several times, then re-bleed and repeat the process until no more air comes out.