OK, I've got a problem with a very soft pedal after bleeding using DOT 5. I wanted to change over to DOT 5 silicon fluid, as the car sits for long amounts of time so I figured silicon fluid would be the most trouble free & maintenance free way to go. I started with a clean dry system, as I rebuilt my SS sleeved calipers with new 'O' ringed pistons & seals from 'Corvette Stainless Steel Brakes Inc'. My new master cylinder came from them as well, so the braking system is pretty much new. All the lines were cleared of the old fluid as were the calipers. The first thing was bench bleeding the MC, which went fine. I also boiled the fluid to purge the fluid of air & let it sit for several hours to cool off before using it. After some time, I (we) finally got the fluid thru the lines and coming out of all the bleeder screws. The problem is that we can't seem to get any pressure on the pedal. The 'pump & crack' method just doesn't seem to be working. I'm sure this's because there's still air trapped in the system. I'm thinking that getting a vacuum pump bleeding system from Advance will take care of this problem. Am I on the right track, or is there something I've overlooked??? I'd appreciate any suggestions & I'm sorry if this subject has been covered here, but I couldn't really find what I was looking for in the forum archives.

 

I've been through this drill with my 78 and essentially replaced/rebuilt as you've described and I use silicone.

The technique you use with silicone differs from that you use with Dot 3...I'm not sure what pump & crack is but if its what I think it is...it's not the technique you wish to use with silicone. The pumping agitates silicone and the problems with air persists...which is why you've got that soft pedal.

The preferred method with silicone is gravity bleed and one at a time. I start at passenger side rear, go to driver's side rear, right front, left front bleeding one at a time. Open the bleeder, drink a six-pack (well maybe not exactly but you get the idea) and come back. Yes this will take a while but pedal on my 78 has been rock hard since 03 when I last bled the system.

Try it and let us know if that helps.

 

I also went with the DOT 5 fluid on my 78's new brake system and gravity bleeding worked great. It seemed to take longer for the fluid to get to the calipers then DOT 3 or 4. I also used a rubber hammer to tap the calipers to dislodge any air bubbles while gravity bleeding.

 

Graity bleed or Motive Pressure Bleeder

 

Works great with Dot 5. As others have said, you need to take care when bleeding (and filling the master cylinder) to avoid bubbles. If you don't have a Motive bleeder, use VERY light pressure on the pedal when bleeding to avoid foaming the fluid. With this knowledge, you can do it either way sucessfully.

 

I never heard of gravity bleeding -------- you learn something new every day. I will be trying this method in the very near future.

 

Shortly after I purchased my '70 I rebuilt the brakes and filled the system with silicone fluid and experienced the same symptoms you describe when attempting to bleed the system using conventional methods.....I then bled the system using a Mityvac vacuum pump and all was well. I have since changed out both front calipers and the right front brake line and re-bled the system the same way with good results. Others on this forum swear by pressure bleeding methods; it seems that for some reason you just can't bleed C3 brakes like you can most cars using the "pump and crack" method- you have to either pull the fluid through the system with a vacuum pump or push it through with a pressure pump.

 

OK, I appreciate all the responses............I've got the vaccum pump kit. Now I need to use the pump on each caliper one at a time , & of course making sure that the fluid in the MC stays topped off. I'm assuming the order would be RR (inner/outer), LR (same), RF & LF. Please correct if I'm incorrect! Now, do I need to re-boil the fluid (not fun) and can I re-use fluid that's already been pulled through each caliper............

 

Never reuse brake fluid that you bleed out as it will pick up all kinds of crud on the way to the bleeder screw. Pitch it and refill the master with fresh fluid.

 

I've never boiled my brake fluid (at least not when it wasn't in the car)- my wife wouldn't appreciate that. As far as the sequence that you suggested, that's been the conventional wisdom so you'll be fine and yes, do toss out your old sludgy fluid. I know silicone fluid is pricey but not enough to be re-using it...

 

I run silicone fluid and even ATF through a coffee filter and recycle it that way....never an issue....

and to aid bleeding without a pump/pressure device....I find drilling some small holes in top of each cylinder cap section...about 1/16th...and installing over a topped off Mcyl....and then put your air jet comprssor into the top, i'ts above the gasket, so the gasket expands down and forces fluid through the system.....when finished it's a snap to just RTV the holes shut for next time....

 

[QUOTE=mrvette;1558292837]I run silicone fluid and even ATF through a coffee filter and recycle it that way....never an issue....

That's a great idea!!

Anyway, I tried the vacuum pump method with a kit from Advance Auto. What a piece of s**t!! The plastic collection cup is sealed with grease, not an 'O' ring, so that's a joke & the little rubber elbow that fits over the bleeder valve is sealed with grease (to prevent leaks) & that's a joke as well. Needless to say, I didn't make any headway using this piece of crap kit. Too bad someone doesn't make a fitting that's hollow that will screw into the bleeder valve opening for a tight fit & then you can slide the rubber tube over that. But, a friend of mine has a Snap On vacuum bleeding kit that I'm getting tomorrow morning & I'm sure (& he says)it works like a champ.

One other question regarding the gravity bleeding method; I've seen alot of posting on how to, which's simple, but I haven't seen where it explains how to know when to stop & close the bleeder valve. Just how much fluid has to drip out before that caliper is done?

 

[QUOTE=MarkV;1558294104]This is how I do it. Starting at the right rear, then left rear, right front, and left front, I open the bleeder screws until fluid starts dripping out with no air bubbles. Close them off, then check the fluid level in the master cylinder, top off if necessary .Then tap the caliper with a rubber hammer, open the bleeder screws again and let it drip until no air bubbles. Do this at each caliper bleeder screws. I still like to do the press, hold the brake pedal and bleed at least once at each bleeder screw. It really doesn’t take very long once fluid starts to drip from the bleeder screw. It’s a whole lot harder writing the explanation then to do it.

 

Tried again tonight with no success. This time I used a Mityvac hand pump and the collector cup had an 'O' ring type seal. For some reason I can't get the damn thing to go up to and hold close to 25 in vacuum. Anything less than this & it doesn't want to pull fluid. Also, when I was able to get the fluid flowing, there seems to be an incredible amount of air coming thru. I wonder if this air could be originating from the bleeder valve itself? Just to claify, I'm using the little black rubber elbow slip fitting that fits over the entire bleeder valve. I also tried using the standard bleeder and the bleeders with the built in check valve. I'm pretty fed up with the whole thing & starting to think that this 'silicon fluid' was a very bad idea; but I'm commited now & I have to make this work. I only have 2 other options. Buy a pressure bleeder or dis-assemble the entire braking system, clean everything including all 4 calipers & then start fresh with Dot 4. Not really what I want to do. Tomorrow I'm getting a Snap On Tools vacuum bleeding set from a friend of mine. Maybe this will work. If not, guess I'll go to ebay & purchase a pressure bleeder & try pushing the fluid thru from the MC back. But this makes no sense, as you're using air pressure to force the fluid thru the system; I thought that air was the enemy, especially when using silicon fluid........ Someone give me some suggestions, please!

 

I have the Mityvac and tried it on my 71 with SS brakes and it continually drew air bubbles. I finally came to the conclusion that the silicone fluid was so thick that the vac was pulling air past the seals on the pistons. So I went with a modified gravity bleed then topped it off with a pedal bleed. The key with a pedal bleed is to use heavy grease on the bleed screw threads. You have one person working the pedal and one working the screw. The one working the screw shouts "down" then the pedal guy pushes gently and holds the pedal down. The screw guy opens the screw a very slight amount then when the pedal hits the floor, the pedal guy shouts "floor!" and the screw guy quickly closes the screw. You might need to go around the car a few times but this worked the best for me in the past.

 

ttt

 

PLease think a bit here, with a couple of small holes in the stock lid, ABOVE the rubber diaphram....and the lid securely in place, you can force air pressure of some MINOR amount into the area ABOVE the rubber....and press fluid throug the M/cyl iinto the lines and throug the whole syste, you just have to be sure you not emptying the m/cyl and so forth...but it's a whole lot cheaper than screwing around and spend MONEY on a 'bleeder'.....

 

You're absolutely correct. This would be using the rubber gasket, not the compressed air to force the fluid thru the system. I've got an old lid; I'll have to rig up a couple small fittings to the lid & try it. Sure beats spending close to a 'C' note on a pressure bleeder...........

Anyway, great suggestion.....thanks for responding (again)!

 

I don't know how many times this has to be posted on here but, every time I use my motive power bleeder I kick myself that I didn't buy it sooner!!!

 

Just a thought, I'm getting a ton of air bubbles. What is the best way to check & see if a fitting or line is sucking air?