The use of poorly designed aftermarket parts aside, if anyone has a spongy brake pedal after converting to DOT5 fluid, there can be only one reason: You did it wrong.

You've got to proceed slowly. If you can't or won't do this, then DOT 5 fluid might not be for you.

Here is how I do it and with this process I have a 100% success rate converting 7 cars from DOT3 to DOT 5:

1. Buy the silicone fluid, bring it home, and let it sit on the shelf for a couple of days. Longer is better.

2. Clean out your brake system by whatever means floats your boat. I'm actually pretty cavalier about this step. I've done the alcohol and compressed air thing. I've started with all new parts. Once, just to see what would happen, I just drained the DOT3, blew compressed air through the system and then started adding DOT5.

3. I put a clean length of 3/8" fuel hard line in the master cylinder and then lay the opening of the silicone fluid bottle against this tube and pour slowly, letting the tube guide the fluid without splashing.

4. I gravity bleed all four corners, keeping the master cylinder full by the means just described.

5. If gravity bleeding, by itself, doesn't produce a firm pedal (sometimes it does, sometimes it doesn't), I sweet talk my ever-patient bride into working the brake pedal while I work the bleeder screws. We have a routine..... she takes about 15 seconds to press the pedal and when I give her the signal to release, she takes another 15 seconds to release it.

6. If, after going around to all four corners a couple of times, we aren't satisfied with the pedal firmness, then we call it a day and repeat the process 24 hours later.

7. At 24 hours later, before we recommence bleeding, I take small hammer and gently rap on the wheel cylinders to dislodge any air bubbles that might have formed over night.


This process ALWAYS works for us.

Oh, and about that one car in which I just drained the DOT3 and started pouring in the DOT5? It actually worked fine for a couple of years. That was my bride's track car. She graciously consented to let me drive it once at Laguna Seca where I noticed the only ill effect of having a few globules of DOT3 still in the system: I boiled the brake fluid coming into turn 11. As soon as the fluid cooled, the pedal was fine again and we drove it like that for another two years until I completely redid the brakes with new calipers and bigger rotors.

Anyway, bottom line, if you have a spongy pedal with DOT5, you weren't careful enough doing the conversion.

Jim

 

I can understand a residual pressure valve being used to hold a slight pressure but not a proportioning valve in a brake system. But neither type valve is needed on a C1 brake conversion (in my view and usage - as well as others who have made the switch). And a proportioning valve is certainly not needed to obtain a firm brake pedal.

Plasticman

 

I have neither on the disc/drum brake setup on my '61 - I did have a proportioning valve on my '66 Mustang conversion though to tweak things for autocrossing...

 

I also have changed over a few vehicles, and "back then", we were not particular about flushing the entire system of the old DOT3 fluid. We just bled new Silicone DOT5 through the system until it ran clear, and buttoned it up.

Had 1 vehicle 18 years with no brake maintenance whatsoever (other than popping the m/c to see if there still was fluid and it was always clear and full). It was a few low mileage vehicle, and the Silicone DOT5 fluid did it's job well. My wife's vehicle also received the DOT5 fluid early in it's 17 year life (using the same bleed until clear fluid came through process, although it did get new front brake cylinders at the same time). I did have to replace the m/c much later, but that was not an issue with the silicone as far as I could tell.

Note that I did use a low pressure power bleeder on the Avanti, but that was the only vehicle that required it (and was necessary using any type of brake fluid due to the design of the braking system). All the others were vacuum bled. Our drag race car also saw the DOT5 fluid. Did several friend's vehicles and never heard a bad word from them. They always bled to a hard pedal.

Plasticman

 

I've run into the need of a residual valave on race cars, when the master is mounted lower than the calipers. You get a drain back and the first stroke of the pedal moves fluid back to the calipers, the second is much firmer. A 2 pound residual will do the trick for most situations, the C-1 should not suffer from this problem.

We have deen around the bend on the proportioning valve before, I have one on the 62, came with the Master Power kit that also had a new dual MC.

Let me see if I understand the use of the PV. Disk brakes operate at much higher pressure, can go up to 1400 to 1600 psi. Drum brakes on the other hand operate at much lower pressure.

So in the event of a panic stop which will cause pressure to spike, and can lock the rear brakes, I think the PV is the correct way to do the job.

Hey, GM used them why not me

 

Frank and Muncieman,

I can understand & appreciate the usage of a proportioning valve, but just don't see the need for it on our C1 Vettes (especially for the way we normally drive them). Just have to be careful in wet weather, and leave plenty of space for braking. In dry (as I have tested several times), they stop very straight with minimal rear lockup. Even on gravel they stopped very straight (all 4 locked up), so I proved to myself that I do not need a proportioning valve (others may take that for what it is worth).

But to need one for getting a hard pedal, noooo way - give me a break (& a brake).

Plasticman

 

The line pressure you cite for disk brakes just doesn't sound right to me.

The brakes on my vintage racer, 4 wheel disks BTW, use wheel cylinder pistons and master cylinder pistons of conventional size. This brake system is designed to produce well over 1G deceleration (tire limited) with a pedal effort of no more than 100 lbs and a line pressure of no more than 525 PSI.

I can't speak to the line pressures in drum brake systems. Drum brakes being somewhat self actuating, the actual line pressure for a given decel rate must be determined by a somewhat involved calculation (or, pragmatically, by an in-line pressure gauge).

Jim

 

I think Jim Lockwood is right on the money for normal brake line pressure for 4 wheel disc brakes imho.

On my A/Gas car with 4 wheel wildwood disc brakes (and a brake pressure gauge) I could get about 3-400 lbs of brake pressure by pushing on the pedal once. After setting the line lock I would pump up the front brakes to 700lbs of pressure to fully stage the car, which was done by skidding the locked front tires forward into the full stage beam by slipping the clutch.

 

I will counter that with some tests I did on our Avanti (Dunlap calipers). We were disapointed with the supposedly "superior" disc brakes on our Avanti (back in 63 they were one of the best available, but by the 70's, they were really dismal).

So thinking we could improve them using an old Corvette trick, we changed the leverage point at the pedal (in other words, we drilled another hole that allowed for more leverage to the master cylinder, at the expense of more travel). It did very little! We then were guess at what was occurring, so I hooked up a 3000 lbs. gauge at the bleeder outlet. Damn near pegged the gauge at very high foot (very extreme panic) pressure.

But learned why we were not getting better braking at the higher pressure (that the altered pedal leverage should of given us). The caliper casting was flexing. In other words, the casting can only take so much before the extra pressure is no longer being transmitted into the pads and rotor. Learned our lesson and put the optional 68 Dodge 4 piston (Kelsey Hays) calipers on there. Braking was a dream after that (and used the Avanti to tow our 23 foot boat besides).

Plasticman

 

I have the Master Power disc brake conversion on my 61". After months of driving, the pedal began losing effectiveness and became "spongy". No amount of flushing and bleeding fixed the problem. In discussion with Master Power about the problem and the obviously bad MC, the technician suggested that I check to see if the locking nut on the rod from the brake pedal to the MC had come loose and allowed the rod to rotate, thus affecting how far the rod was moving when I applied the pedal. That was it!!. I turned the rod enough turns to get the pedal feel correct and tighted the lock nut back down. Worked perfectly and still does. Bert.

 

Can you show us a pic of your install? Caliper bleeder position?
brgds
rene

 

The only thing that causes a spongy brake pedal, assuming that the pads are parallel to the disc , the flex brake lines are not stretching and that there are no hydraulic leaks, is air in the system - period. Simple 8th grade science truth is that liquids are not compressable but that gases are. You have air in the lines somewhere that you need not to have. The master is from what vehicle? I know that the one that I used in my C2 is the 67 C2 dual master and that it absolutely had to be bench bled. I also know that you need a check valve (residual) in the system or the pistons will retract in the calipers and that , depending upon the type, they can suck air in when they do.

Brakes are pretty simple things, especially disc brakes. You step on the pedal and use the mechanical advantage of a lever to transmit force through the liquid to the disc. Air causes the force to be delayed while it compresses.

As I re-read this I have to ask an obvious question, the seals in the master and the flex lines are compatible with DOT5, correct? I recall someone using aircraft fluid in his car some years ago and it toasted all of the rubber seals..

 

Drum-drum systems don't need proportioning valves. Disc-disc systems don't need prop. valves. Disc-drum systems use them because of the self-energizing characteristic of the (cheap) drums we use. (If you double the line pressure on a disc brake, you'll get double the brake torque. If you double the line pressure on a drum brake, you'll get more than double the brake torque.) The prop. valve limits pressure to the drum rear so you don't get premature rear lock-up. You could limit rear lock-up with smaller wheel cylinders, but at normal braking levels, you'll wear the fronts out much quicker than the rears. That's why GM uses a prop. valve.