Disc brake troubles

[Thane63]

I need a little help with my brakes. I installed a dual master cylinder from an early 70's Dodge Dart on my 1963 Avanti with factory disc/drums. I removed the front residual valve from the m/c & bench bled the unit. I also adjusted the power booster to match the new m/c. I rebuilt & cleaned out all wheel cylinders & lines with denatured alcohol & proceeded to bleed the system with DOT5 silicone brake fluid.

The rears bled just fine, but I couldn't get anything to flow from the front discs using the two man bleeding method. I have a vacuum pump so I bled the fronts using vacuum. I had a nice firm pedal & thought all was well. Next morning they were soft, so I bled the fronts again manually & hardly anything would come out of the bleed screws. I bled the fronts manually for 20 to 30 cycles with very little fluid coming out of the bleed screws & no air visable. It had a nice pedal, so I drove it that way a couple days & each time I would drive it the pedal would have a different feel. Sometimes soft & sometimes hard. Yesterday it felt really firm & I thought they were magically coming around some how. Well after about 10 miles down the highway I found out that they came around too well. The fronts were now dragging to the point of actually smoking when I came to a stop. I shut everything down & let it cool, hoping I didn't warp the rotors. When I tried it again the pedal went to the floor.

What do I do now?

Thane

[Bud]

It sounds to me like you have a defective master cylinder. I just had that happen to me with my Avanti dual master cylinder only the front section of the cylinder wouldn't pressurize. I replaced the cylinder under warranty and now the brakes work fine. Also be sure that you have installed the lines correctly as there is only one way that they should be connected. Bud

[Dick Steinkamp]

Are you sure the MC you installed has the same piston(s) diameter as the stock one? Will the holes to the reservoir uncover at the same time as they would with a stock cylinder (for both front and rear chambers)?

Is the push rod adjusted to spec?

Stock Studebaker disc brakes did not have a residual check valve in either the front or the back lines. Stock rear brakes on a disc brake Studebaker are not like normal drum brakes.

Mixing brake parts is a crap shoot. I know the downside of a single cylinder MC, but IMHO the downsides of mixing in a brand X MC can often create even more downside (unless you are a hydraulic engineer or have done extensive testing).



Dick Steinkamp
Bellingham, WA

[Thane63]

Thanks for the help, guys.

The front brakes are tied to the aft larger reservoir & the rears to the front. I tried removing the rear brake residual valve, but couldn't get them to bleed manually, so I re-installed it & instantly had fluid the next time I cycled the brakes.

I took my m/c & brake booster to a local long time brake specialty store & they set me up with the m/c & rebuilt the brake booster. I told them what I had for brakes & he dial indicated the factory m/c bore at the counter. I guess I assumed he knew what I needed. Maybe I assumed wrong.

Sounds like a defective or wrong m/c or improperly adjusted brake booster.

Thane

[Gunslinger]

Several possible problems...

1) You might have bad rubber brake hoses. They might look good externally, but could be deteriorated internally and collapsing. If they're older hoses (even original), replace all on the car just for general principles.

2) You have a front caliper not leaking, but sucking air in. I had this happen on a Corvette I owned. The calipers were rebuilt and no more problem.

3) You haven't completely flushed the system of DOT3 brake fluid before adding DOT5 synthetic. They are completely incompatible and cause similar problems as you describe. Check inside your master cylinder...if you see any cloudiness the fluid is contaminated. Synthetic fluid is difficult to fully bleed air out of and any air bubbles like to hide in any curved or angled areas of the hydraulic lines. Synthetic is also not compatible with some rubber components of the hydraulic system. Modern seals have no problem...if the rebuild kits you used are older, it could cause a problem, but not likely this quickly.




Poet...Mystic...Soldier of Fortune. As always...self-absorbed, adversarial, cocky and in general a malcontent.

[Thane63]

5 years ago I replaced all lines including the rubber brake hoses & had all the wheel cylinders re-sleeved with stainless. This time around I blew out all the lines & hoses with compressed air, ran a quart of denatured alcohol thru, let sit for a while, another quart of alcohol, then blew the lines out with compressed air & let sit for 24 hours. I then replaced wheel cylinder kits on all four wheels, filled keeper spring cavity on the front pistons with DOT5, double checked the front caliper alignment tolerances & proceeded to bleed. Maybe I missed a step.

I'm hoping I just figured the brake booster reach rod incorrectly. Tonight after work I will separate the m/c from the booster & double check the measurements.

I leave for Cedar Rapids in a week & brakes would be nice for the trip.

Thane

[WCP]

I believe alcohol is a poor choice to use for line flushing. It is not sufficiently volatile to dry off properly from the lines inside surfaces and is very hygoscopic. The compressed air will surely introduce moisture into the lines that will probably not be completely purged during the bleeding process. This may be contributing to your problem.

[bige]

Did you bleed the master cylinder? You need to bench bleed it by itself running hoses from the outputs into a jar of fluid or even back into the reservoir itself.

ErnieR


On its way to a 15.097 Island Dragway Great Meadows NJ Spring 2006.

[52 Ragtop]

"Raybestes" states that rubber brake lines should be replaced every 2-3 years! Just because you replaced them 5 years ago, does NOT mean they are still good!
I have run teflon lined braided lines on my cars, BUT,they are NOT DOT certified for street use! They will stop Indy cars from 200 MPH, but not DOT approved! They do not pass the "whip" test!
One other question, Is the M/C you purchased new? or rebuilt? I will NOT use or sell a rebuilt M/C, they are not rebuilt properly and I have had a few that would do the same as you are indicating.

Jim

[Thane63]

I separated the m/c from the booster last night & saw immediately what the problem was.

The booster reach rod was way out of tolerance.

I had the booster rebuilt & I had set the reach rod at that time. I later found out that the booster had a leak around the area that the two halves crimp together, so I took it back to the shop & they cinched it up. I neglected to recheck the reach rod measurement & found out last night it was long by 0.15".

Thanks again for everyones help.

Thane

[Bellingham Studenut]

It's important when changing to a different master cylinder to have the reach rod adjusted, or it will cause your brake pressure to braking all the time and is more noticeable in front than back (60/40%).
I have had the same happen and you eventually get to an expansion, or boiling point wondering why your needing more gas pedal (not rolling so easy) and getting more fluid in the reservoir.
A quick adjustment, or two and you can set it fine.
If you find a good place to safely slide the car with fast braking, you can tune/even brakes by adding a adjustable proportioning valve and getting front & back to lock up evenly.
Braking distribution was made for overall application, just like a stock cam shaft, so you can fine tune it for specific applications if needed.
I use a proportioning valve to fine tune and adjust my hauling Champ truck brakes differently when loaded & trailer, than just the empty truck.

DOT 5 fluid is great, but best used when replacing everything in a clean new system.
It does seep out easily if any pistons (master/wheel cyl, calipers), are worn, or pitted. A few fittings usually also need retightening after filling with silicon fluid the 1st time, but dot 5 will keep new parts working well for many years.

James


The Bell Collection
Bellingham, WA.