3/4" master cylinder?

[goldduster318]

Well there's another clue. DOT5 has a higher compressibility and won't absorb, move, and then dissapate the air the way DOT3/4 will. I will only personally ever use DOT3 or DOT4 and when I am working on the car I just would be ready to pour some water on it. The DOT3/4 is a higher performing fluid.

You'd have to work pretty hard to change between the two because mixing them is well...horrible.

My job being a brake engineer for ABS/ESC systems and electronic brake boosters we don't allow that stuff (DOT5) to be used for any reason. Its not an issue with EPDM rubber but the stuff will just straight foam up if it's agitated enough. This could also be part of the issue you have. When you have air in the system you're basically foaming it and then you have to push it out now...without foaming it.

DOT3/4 if you start with an empty system, foam it, let it sit overnight and then do a quick bleed you're usually set regardless of how bad it was because the air makes it to the highest point it can reach and comes out of it quite fast.

 

[Icetech]

Well there's another clue. DOT5 has a higher compressibility and won't absorb, move, and then dissapate the air the way DOT3/4 will. I will only personally ever use DOT3 or DOT4 and when I am working on the car I just would be ready to pour some water on it. The DOT3/4 is a higher performing fluid.

You'd have to work pretty hard to change between the two because mixing them is well...horrible.

My job being a brake engineer for ABS/ESC systems and electronic brake boosters we don't allow that stuff (DOT5) to be used for any reason. Its not an issue with EPDM rubber but the stuff will just straight foam up if it's agitated enough. This could also be part of the issue you have. When you have air in the system you're basically foaming it and then you have to push it out now...without foaming it.

DOT3/4 if you start with an empty system, foam it, let it sit overnight and then do a quick bleed you're usually set regardless of how bad it was because the air makes it to the highest point it can reach and comes out of it quite fast.

yeah.. i did some research and dot5 seemed like it would be ok.. and i know i somehow always spill brake fluid so i also figured it would save paint I would have to swap pretty much everything to switch to 3, from what i read anything rubber will expand and fall apart if you mix 3 and 5

 

[Icetech]

Well there's another clue. DOT5 has a higher compressibility and won't absorb, move, and then dissapate the air the way DOT3/4 will. I will only personally ever use DOT3 or DOT4 and when I am working on the car I just would be ready to pour some water on it. The DOT3/4 is a higher performing fluid.

You'd have to work pretty hard to change between the two because mixing them is well...horrible.

My job being a brake engineer for ABS/ESC systems and electronic brake boosters we don't allow that stuff (DOT5) to be used for any reason. Its not an issue with EPDM rubber but the stuff will just straight foam up if it's agitated enough. This could also be part of the issue you have. When you have air in the system you're basically foaming it and then you have to push it out now...without foaming it.

DOT3/4 if you start with an empty system, foam it, let it sit overnight and then do a quick bleed you're usually set regardless of how bad it was because the air makes it to the highest point it can reach and comes out of it quite fast.

Just did some reading after what you posted.. didn't know bout how bad dot5 likes to hold air.. guys say it takes time to work out and just bleed and let it sit and bleed again and they will work out.. soo that is prolly my issue.. will just work it out over time now that it's safe enough to drive at least

 

[Bewy]

Ice,
I would weld a brace across the weld you did on the pedal. Weld looks ok, but there is a large amount of force on that weld in a panic stop.

 

[Icetech]

Ice,
I would weld a brace across the weld you did on the pedal. Weld looks ok, but there is a large amount of force on that weld in a panic stop.

next time i have it out i will...

 

[Alfajim]

I'm not actually clear that when you get 1000 psi or 650 psi, that everything including the caliper is "in the system". The pressure should be the same everywhere. The thing is there is an input/output force going on here so how much force is being applied to the pedal itself has a lot to do with the output.

650 psi is like 44 bar, this won't lock the wheels, that's clear.
1000 psi is 69 bar, probably enough to lock them on gravel, but barely.

Usually a car with really effective brakes could have a locking pressure of about 80 bar / 1160 psi and one that's not that great or has big tires could be about 120 bar / 1740 psi.

The changing of the pedal ratio just by moving the connection point to the pedal is a horrible idea, the piston rod on the master cylinder should never have an angle at any time in it's stroke more than 3 deg above or below the axis of the piston bore or it will wear out. You'd need to do it by keeping that constant but changing the pedal length or moving the pivot itself...or you'd have to move the MC on the firewall.

I would still think you have air in the system, put a block of wood in place of the rotor and make sure the bleeder is absolutely the highest point.

The other thing would be, are the rear brakes adjusted?

That block you have does nothing, I would just union the rear lines together and use a regular T on the front ones. To me its pretty bizarre that it's even made the way it is. Also remember that the MC port closest to the firewall is for the FRONT brakes and the port closest to the front of the car is for the REAR brakes.

It appears to me that what we have here is a hydraulic ratio problem. The 2.75 dia. caliper piston requires a lot of pressure to get a good clamping of the pads to the disk and that m/c size tells me it should have a power brake booster to get there. I have worked on a lot of import disk brake cars, Jags, MG's, Triumphs, Ferrari, Maserati, Lamborghini, etc. They all had very small master cylinders. Take Jag E type it used two piston calipers all around 2 1/4 front and 1 3/4 rear with two master cylinders of 5/8 dia. So there is a Hugh pressure ratio here and it is amplified with a booster that adds force to the pedal. there is also a balance bar for equalizing the pressure between the two m/c's. It looks like a booster is required or a much smaller master cylinder to increase the line pressure. The 11/16 dia. cylinder should help here.
Also make sure that there are no upses downses in the lines especially the rear one and no where is any part of the system higher than the master cylinder to trap air.

Jim O

 

[Bewy]

The extra pressure to apply the pads comes increasing the diam of the cal piston....not by adding a booster [ although it is a good idea ].
When you increase piston diam, you are increasing the area that the fluid pressure applies against. If one piston has a 2 square inch area & the line pressure is 1000 psi, pressure on the piston is 2000 psi. If piston area is increased to 3 sq in, pressure on the piston is 3000 psi.

 

[92b]

The extra pressure to apply the pads comes increasing the diam of the cal piston....not by adding a booster [ although it is a good idea ].
When you increase piston diam, you are increasing the area that the fluid pressure applies against. If one piston has a 2 square inch area & the line pressure is 1000 psi, pressure on the piston is 2000 psi. If piston area is increased to 3 sq in, pressure on the piston is 3000 psi.

It also requires more fluid volume to move the piston the same distance. If you keep making the master smaller and the pistons bigger or increasing the pedal ratio eventually you end up with a pedal that has a lot of travel. A booster is a way to decrease pedal effort or increase pressure for the same effort without increasing pedal travel.

 

[Bewy]

92,
I covered fluid volume weaaaaaaay back in post #30. Pedal travel is only decreased if the m/c diam is increased.

 

[92b]

92,
I covered fluid volume weaaaaaaay back in post #30. Pedal travel is only decreased if the m/c diam is increased.

Yes, you covered how the master cylinder affects pedal travel in post 30. I added how caliper size and pedal ratio affects travel. And how a booster can add pressure with out changing pedal travel. For those who may be interested.

 

[Jim Kueneman]

Want a Chinese booster that has a CRAP load of boost. I took it off my 63 with Kelsey Hayes dual pistons and put a rebuilt 64-66 factory booster. It was too touchy for me. Free with shipping. You can have to MC too. 15/16” bore.

 

[Inertia]

Here's the calipers that are my favorites, 89 Asian (Mazda RX7) finned aluminum 4 piston calipers, weigh half of Kelsey Hayes or single piston cast calipers.
Pistons are 1.75 ish near same as K/Hayes.
$50 - $100 each at the wreckers or eBay .
Unsprung weight is very important in the cars I build.

Know what size Masters they use ? - 3/4, or 7/8" from factory.

As mentioned by Alfajim - you don't need a ton of volume of fluid, with or without power booster, just a lotta line pressure .

P.S.

I also put the 15/16 master on Chargers, Challengers, Roadrunners, GTX . . .

 

[92b]

Here's the calipers that are my favorites, 89 Asian (Mazda RX7) finned aluminum 4 piston calipers, weigh half of Kelsey Hayes or single piston cast calipers.
Pistons are 1.75 ish near same as K/Hayes.
Unsprung weight is very important in the cars I build.
Know what size Masters they use ? - 3/4, or 7/8" from factory.
As mentioned by Alfajim - you don't need a ton of volume of fluid, with or without power booster, just a lotta line pressure .

View attachment 1716450449

What spindle and caliper mount bracket do you use?

 

[Inertia]

I fabricate them, - since the 70's, - on Super Modifieds, Late Model Stocks, whatever my passion .
I come from the age where we made ****, - not bought from a catalog.

 

[92b]

I fabricate them, - since the 70's, - on Super Modifieds, Late Model Stocks, whatever my passion .
I come from the age where we made ****, not bought from a catalog.

Do you recall using those calipers on stock Abody spindles and rotors?

 

[Inertia]

Do you recall using those calipers on stock Abody spindles and rotors?

No, I have not.
I've never found a need, but I know some members here were working on some with 6? piston benbos using reg Mopar hubs for autocross perhaps.
Really interesting, and radial mounts, coulda used those on our last car.
Had to stuff those calipers above, and Mini Cooper rotors inside a 13" slick.
Won half the trophies avail that year (2022).

Its not hard, there is a listing of "hats" by dimension/offset, etc
Figure out which hub, - where the rotors needs to sit, figure out which offset, modify/fabricate mounts as nec.
Drill/machine "hat" to bolt pattern.

Good luck .

 

[Icetech]

Here's the calipers that are my favorites, 89 Asian (Mazda RX7) finned aluminum 4 piston calipers, weigh half of Kelsey Hayes or single piston cast calipers.
Pistons are 1.75 ish near same as K/Hayes.
$50 - $100 each at the wreckers or eBay .
Unsprung weight is very important in the cars I build.

Know what size Masters they use ? - 3/4, or 7/8" from factory.

As mentioned by Alfajim - you don't need a ton of volume of fluid, with or without power booster, just a lotta line pressure .

P.S.

I also put the 15/16 master on Chargers, Challengers, Roadrunners, GTX . . .

View attachment 1716450449

I wish i could have gone with those... very cool and good price

 

[Icetech]

Just wanted to update.. speed bleeders came in and i bled again and the brakes are quite a bit better.. so i think the dot5 was the issue.. i'm going to bleed them periodically till all the air works through the system, those bleeders make life easy...

I can't lock them up but i'm also not afraid i won't be able to stop.. so it's much better.

 

[Inertia]

Thnx for letting us know how it turned out.

 

[Icetech]

Thnx for letting us know how it turned out.

yeah i always hate finding a old thread and its like "I fixed it" with no explanation

 

[Bewy]

I do not think the type of brake fluid was the issue. Brake hydraulic action works the same for all the fluids. Glad you got it sorted.

 

[Icetech]

I do not think the type of brake fluid was the issue. Brake hydraulic action works the same for all the fluids. Glad you got it sorted.

Did research.. dot5 has a issue with holding air (from what i have read) i ended up bleeding.. letting it sit for a day or 2 and bleeding again.. did it like 4 times and they are decent now.. no idea if that is 100% the issue or was just some air trapped somewhere but at least they work now

 

[Newbomb Turk]

Did research.. dot5 has a issue with holding air (from what i have read) i ended up bleeding.. letting it sit for a day or 2 and bleeding again.. did it like 4 times and they are decent now.. no idea if that is 100% the issue or was just some air trapped somewhere but at least they work now

Well hell. I didn’t know about D5 holding air either. I was going to convert my garbage over to it but now I’m not sure.

 

[JDMopar]

Can you flush the system with denatured alcohol and go back to Dot 3 fluid?