Do I need to bleed the fronts if I only open a rear line?

[chrisbruck]

I'm going to replace the proportioning valve on my rear brake circuit in my stock '87 tonight. It's in the trunk and connected in-line on the rear line.

After doing this, should I have to bleed the FRONT lines again? Just trying to plan my time tonight and fluid qty needed.

[DanielDudley]

If you are careful about keeping the line from bleeding empty, you should only have to do a minimal amount of bleeding.

Keep the reservoir full, crack the lines on the old valve and then disconnect the valve. unscrew the lines to the valve and immediately hook up the new valve. Let the fluid fill up the valve before you tighten down the fittings, and then bleed the system.

If your old fluid is more than 2 years old, you might as well bleed the fronts anyway.

[chrisbruck]

Cool, that's what I was planning on doing... I just bled the system a couple weeks ago before finding my "MacGyver'd" rear brake line. Putting it back the way it's supposed to be and figured the front would be fine as it's completely separate.

[Driver_X]

This is just my opinion, and you're probably right, but what's the harm in bleeding the fronts just in case? I never like taking potential shortcuts regarding brakes. Again, you're reasoning is sound, and I get it, but for the time it takes, why not?

[chrisbruck]

I plan on doing the fronts again this weekend just to be safe.... if I don't "have" to do them tonight then I will just wait. This way I can just replace the valve, jack up the back and bleed the back right quick.

[CCM911]

I had to replace the proportioning valve on my 1984. I think I ended up doing a full bleed just to be sure.

Why are you replacing? In my case, the rears would grab first and try to snap out the rear end.

Best fix I ever did.

[chrisbruck]

Some genius "prior owner" removed it completely.... never had the rears lock on me and spin but I can really feel strong rear bias on heavy braking. I'm hoping the overall feel will balance out better after I put the rear P/V back in place like the factory did it.

[Charles Freeborn]

Technically no - the rears shouldn't affect the fronts... but....unless the fluid is less than a couple of years old, while you've got all the tools and mess out why not flush and bleed them all. Changing brake fluid is one of the simplest and cheapest upgrade/maintenance items one can do. The loud pedal is fun, but the middle one is more important...
-C

[mreid]

Depress the brake pedal about an inch and brace it in that position (I used an adjustable paint roller handle). This keeps fluid from flowing. Remove the offending device, seal up and only bleed the rears. I just replaced my rear calipers and this method worked great.

[chrisbruck]

Did what "mreid" said and it worked like a champ.... a tad spongy still but will do a full bleed again this weekend. Putting the P/V back on that was not there before seems to make a nice braking difference. Feels like a little more balance back on the front end. Car seems to stop a little quicker too. Could all be in my head though... at least now I feel good it's the way Porsche designed it.

[chrismorse]

Originally Posted by mreid

Depress the brake pedal about an inch and brace it in that position (I used an adjustable paint roller handle). This keeps fluid from flowing. Remove the offending device, seal up and only bleed the rears. I just replaced my rear calipers and this method worked great.

It might take more than an inch to let the MC seal move past the brake ports, I'd go a bit further just to make sure that the fluid doesn't run right out. This should prevent too much fluid loss, that would necessitate the bleeding of the fronts.

I'm with the other guys in bleeding the fronts and flushing the fluid if it has been a couple of years or you have seen any track type use.

Going a bit further, I would recommend against doing the usual pedal bleeding Go with a pressure bleeder or a slow carefull vacuum bleed, (low vacuum).

When I was a service manager, I used to be the one volunteered to do the pedal UP-DOWN bleeding. I noticed an unexpectedly high number of master cylinder failures just after bleeding and I think it was old master cylinders that had not been kept clean by annual bleeding that failed.

My theory is that as the MC gets use and stuff ages and deteriorates, the piston only travels a short distance in the bore and so, any debris, varnish/crud/rust gets pushed down the bore to the usual stopping place, you wind up with a gradually solidifying pile of junk at the end of the normal master cylinder travel, (not necessarily the end of the cylinder).

Everybody runs the brake pedal to the floor on manual bleeding and bingo, the old seal scrapes past the pile of solidified crud and the seal gets nicked & the pedal creeps to the floor.

So, if the car is old and hasn't had diligent annual or biannual bleeding, or you don't know the service history, you don't want to push the pedal to the floor.

My .02,
HTH,
chris

[DRACO A5OG]

Sorry to bring this old thread back OP, but any updates or pit falls?

Did you just replace the P/V or the line adaptor as well item #11 on pag diagram.

Mine is leaking at the P/V. Apparently my heavy footed two man brake bleeding method broke the internal seal of the P/V. Guess will stick to old school gravity bleed and keep it simple stoopid

Tia,

Jim