Author Topic: Brake problem (Read 6613 times)

TIPPA · « **on:** September 14, 2005, 01:40:21 PM »

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 · « **Reply #1 on:** September 14, 2005, 03:33:04 PM »

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 · « **Reply #2 on:** September 14, 2005, 04:35:04 PM »

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 · « **Reply #3 on:** September 14, 2005, 07:52:32 PM »

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

VET-005 · « **Reply #4 on:** September 15, 2005, 07:53:47 AM »

Tippa it might be time to upgrade those brakes and put those hedaches behind you.

smokem · « **Reply #5 on:** September 15, 2005, 06:02:08 PM »

may need pressure bleed call me if you think might help

77CVT · « **Reply #6 on:** September 16, 2005, 08:23:35 AM »

Oh, and one other drama I had. The "experienced" large brake recondioner put a couple of the piston seals the wrong way up. Some had the metal face on the inside which of course rusted!

Idiots!

69 DIRTY RAT · « **Reply #7 on:** September 16, 2005, 11:25:43 PM »

TIPPA I have a Brand new preportion valve that you can have (for the price I got it, I think I got it from Summit?) I'm still saving for my brakes so I wont need another til I buy the New Brakes, So your welcome to it if you think it will fit? It looks like it will fit anything??? Call me 0411535160 if you want it?
BUT I think its air gettin in your calipers, MR69er had a similar problem,he changed everything yet it still leaked. Found out one of the new calipers had a slight leak (sucked air in). It would be fine when it was bleed BUT after driving for a while it would get spongee!
or slightly warped rotors,combo????

TIPPA · « **Reply #8 on:** September 17, 2005, 02:00:04 PM »

Thanks everyone for all your advice! Unfortunately Shayne's car has been off the road with me changing its timing belt, water pump and distributor and now has a cracked radiator (cars...;30) so getting her mobile again has taken up all my time ;30
So unfortunately won't have time to fix the vette for Fox ;37 We will come along just to say hi and check it out though.
However will try to have her ready for American Nats and being only next weekend i'll be having plenty of late nights....

Andy if you could bring your proportioning valve (or distribution block thingy) to Fox that would be a huge help and i'll bring some cash with me. Just let me know how much you want for it. Thanks mate!
:)

TIPPA · « **Reply #9 on:** September 18, 2005, 08:43:27 PM »

77CVT, thanks for the hub/disc run-out advice. A few i've now spoken to agreed run-out can allow caliper pistons to suck in air. I'll be getting a dial indicator on tues so will see how far out they are.
:}

TIPPA · « **Reply #10 on:** September 19, 2005, 02:08:57 PM »

Ok i've found the problem... front right caliper is weeping.(i had the rears recoed recently but a misunderstanding led me to believe the fronts were done only a little earlier... was informed they haven't been touched...)
Also measured disk run-out: between 0.060 and 0.15mm measured on disc surface where pads run, from inner to outer cirumference and also along machined hub face where wheel studs are. Is that within tolerance?
In the meantime i'm hurling my front calipers and looking desperately for a trouble-free final solution.
What do you all recommend? I don't want to spend more than, say, AU$500 for the pair.
Ecklers have new AC-Delco for US$150 each, or remanufactured ones for US$109 both types with o'ring seals and s/s sleeves.
Does anyone know of any calipers that i can buy locally so i have a chance of making it to American Nats?
Thanks!

77CVT · « **Reply #11 on:** September 19, 2005, 03:46:01 PM »

.1mm = 4 thou which is as good as you will ever get. No calipers will fit unfortunately. Any o ring, s/steel ones will do the job.

If you get yours redone, don't forget to get the bleed screws redone as they can sometimes suck a tiny bit of air, especially after 20-30yrs. You can get them from Hopper Stoppers for about $8-10 each.

$109 sounds good although freight might kill the price saving. Mine cost me $250 ea to get done here.

TIPPA · « **Reply #12 on:** September 19, 2005, 04:09:54 PM »

Great. Just rushing out to pick up a pair of recos with s/s sleeves right now which will match the back too. Consider myself lucky being able to find a pair ready to go. So American nats here i come ;):)

69 DIRTY RAT · « **Reply #13 on:** September 19, 2005, 05:11:14 PM »

Do you still need the proportion valve?

TIPPA · « **Reply #14 on:** September 19, 2005, 07:10:14 PM »

Probably not thanks 69DR. Mine looks fine and i've found the caliper was the problem so it should be fine now. Looking at chrome bumpers at Fox i saw their proportion valve is slightly different to mine so would have to re-bend my lines to suit which could lead to probs since they are 27 years old...
I appreciate the offer so thankyou anyway :b2
So now i'm looking for quality brake pads. Does anyone prefer a certain brand?
I used to go for Bendix Ultimates in my other cars but assume they dont make them for corvettes. Do we have to get ours relined or can we get them off the shelf here?

ozpont · « **Reply #15 on:** September 19, 2005, 08:24:09 PM »

Hope this helps also.... this stuff is here already.
..www.usgmsp.com

74-82 GM Corvette Proportioning valve.. GM Genuine new..(Not chinese... )THree in stock A$124+ gst delivered.

Front Calipers. COMPLETE with pads, PINS etc.. .. not partial complete.. $350 odd + gst. If you want partials.. $248+gst. Exchange.

Hose kits: $140 for all 4 (all usa made - not indian)
Master Cylinders, calipers, bleeders, rotors, pistons, caliper kits, boosters etc. All in stock from 68 thru 92 era.

.. thats here.. today... and also .. in stock.

TIPPA · « **Reply #16 on:** September 19, 2005, 08:36:50 PM »

Wow those prices are GREAT :22
Thanks ozpont- i'll check them out!

77CVT · « **Reply #17 on:** September 19, 2005, 08:54:32 PM »

I got bendix performax on my '85.

Just don't get ferodo/hardie pads - too harsh and glaze the discs under high braking applications!

TIPPA · « **Reply #18 on:** September 22, 2005, 09:30:49 PM »

Quick update... cleaned and re packed front wheel bearings, had rotors machined that day, painted new calipers and then fitted rotors, calipers, new pads, new flex brake lines, new bleed nipples, and checked disk run-out again all on wed night (till 11:30pm with a 3:30am start for work...). Fitted new brake proportion valve, bought pressure brake bleeder and put her on blocks with all wheels off tonight (thurs). Now gotta buy brake fluid and pressure bleed the brakes in the morning, drive her in for a pink slip at lunch time (rego ran out on wed) and pay rego and green slip in the afternoon ready to drive her down to Canberra for the American Nationals on Saturday!
;););)
It will be a close call so hoping tomorrow goes as planned ;ay

Vette Obsessed · « **Reply #19 on:** September 22, 2005, 10:19:35 PM »

Good luck TIPPA. I've been in your shoes before and it ain't fun.