Technical Tips > Brakes, Suspensions, Steering, Wheels & Tyres

Brake problem

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TIPPA:
I have a problem with my 78's brakes... they don't work. On the drive back from the Brizzy show my pedal started off hard but became more and more spongey untill eventually it was touching the firewall and doing nothing.
My master cylinder is brand new.
My calipers are freshly reconditioned.
New flex lines at each corner.
No external leaks at all.
On recommendation i removed the brake proportioning valve and had a look at it. On removing the hexagonal end i saw a black "button" inside that may activate a switch which comes off the side. There is no springloaded piston or spool or anything else in it that i can see and no corrosion or dramas with it by the look of it. It seems to be just a distribution block with a switch rather than a proportioning valve...
*Just looked thu Ecklers and it IS a proportioning valve, but is classed as "non-repairable". Guess there IS a piston etc inside but switch must be swaged in after it.
Either way Classic Chev are Nil Stock. Chevparts are returning my call later today and Ecklers have them for US$59 but that wont help me for Fox!
HELP ;37
I would be pretty disappointed if a simple problem stopped me from going to both Fox and American Nationals ;30
Any advice on whether this part could be the culprit or if i should look elswhere will be greatly appreciated!!!

TIPPA:
Ok, Chevparts nil stock too but Corvette Alterations and Conversions have one (got their add from our mag). They believe its unlikely to be the proportioning valve, and most likely be either a caliper piston seal sucking air (apparently they are temperamental and are known to suck air without leaking fluid) or a disc rotor runout problem (if a rotor is warped slightly it will push all 4 caliper pistons further into the caliper so it requires more fluid to push them back out to clamp the rotor. When the brakes are applied the pedal won't pulsate either, like most cars as the 4 pistons will move in and out from one side to the other, which makes troubleshooting a little more involved).
He offered a simple technique to check for air in the system which is so obvious i've never thought of it ;39
Simply clamp all 4 brake flex lines and push the brake pedal.
If its spongey there is air from master cylinder to flex lines...
If its firm, there is air in one or more of the calipers!
Remove one clamp at a time and which ever one causes a spongey pedal pin points the offending caliper!
Now where are my vice grips...:22

77CVT:
Welcome to the world I was in when I had my '77!

That is the best way to find which corner is the offending one.  I am willing to bet that if your discs are new, your calipers are reconditioned that it is your hub.  Get a dial indicator and check the runout.  If it is out you can take it off and get it machined true.  My rears were out by 10 thou and that was enough.  For the fronts I shimmed it until the best I got was 4 thou runout.

The block is not a proportioning valve.  The proportioning is achieved by the different sized calipers front to rear!  All the block is supposed to do is close off whatever end is failing.  Here is a copy of an e-mail I got from Tom (forgot the last name) over at the Corvette Forum some time ago.  He removed it to see if there was a difference but I never saw the result.

"Chris...   Jenny & I did indeed complete the removal, disassembly, and
cleaning of the valve on my 70 and her 80 Vette. We have pics.

We have one more test to run, and then we will post the results. Bottom line....  the valve can mess you up big time when trying to bleed the brakes.

We had to bleed both a rear AND a front caliper at the same time to avoid causing the valve to move off center, and on the 80, the only way we could get it to re-center was by blowing compressed air backwards through the brake line. The 1980 valve was clogged by DOT 3 gunk so bad, that it would move off center and get stuck. The 70 valve was much easier to work with.

Jennys car now has good brakes, but I think she also needs a new booster. It will have to be a Christmas present because I already gave her her birthday presents.  LOL

Steps we followed:
1) rebuild/replace the master cylinder...  at minimum... disassemble it & clean it out.
2) disassemble all 4 calipers & clean them out.
3) if the caliper seals are old, replace all of them
4) install new hoses.... we used rubber at first, but then switched to SS braided/teflon
5) remove, disassemble, and clean the valve... VERY carefully (let me know if you need more info)
6) re-install all components
7) bench bleed the M/C with special tubes (let me know if you need more
info)
8) test the M/C with special plugs (let me know if you need more info)
9) remove plugs, install lines.
10) bleed one front and one rear caliper simultaneously
11) bleed the other pair.

It takes a lot of fluid to fill the calipers...  it takes quite a lot of pumping. If one of the open lines bleeds much faster than the other, the valve is off center. You need to re-center it to finish the job...  maybe several times.

The method of valve re-centering depends on the year/design of the valve.

Tom"

What I also did with my '77 was to take the springs out from behind the pistons.  This stops them being pushed against an untrue disc.  I never got to do it to the rears though.

I ended up buying 2 Wilwood calipers and I got some plans for aluminium brackets to fit them but as usual....I never got around to it.  :b2

Get a dial indicator and check your runout!

BrianVette:
Tippa I hope you get it sorted , cant miss Fox and look forward to seeing you there.;29;29

VET-005:
Tippa it might be time to upgrade those brakes and put those hedaches behind you.

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