Trying to figure out brake problem with rear disc upgrade

[Apogee]

Originally Posted by PRRC

I would say that 1 problem with this set up your running is the 1/4 line to rear calipers. It's just to large of a diameter for a disc. set up.

Expansion due to pressure within a brake system is referred to as system compliance. All components have some compliance including calipers, hoses, hard lines, master cylinders, etc, however the goal is to typically minimize it as much as possible for the most responsive braking system possible. When comparing various components, rubber brake hoses are by far the largest contributors to system compliance, so if you have rubber brake hoses, it doesn't really matter what the other components are since their contribution is negligible.

If you want to look at the difference between 3/16" and 1/4" brake tubing, it's just a matter of comparing the hoop stresses and associated strain values. Assuming the wall thicknesses are the same (typically .028") and so is the tubing material, the 1/4" tube will expand 45% more than the 3/16" tubing if my math is correct. Considering how little the 3/16" tubing expands under pressure, 45% more than that is still a basically negligible IMHO.

On most street cars, I can't tell the difference at the pedal if a car has 3/16" or 1/4" tubing. If you had a car set up for track use that was right on the edge of locking up the rear tires at threshold braking and you made the swap from 1/4" to 3/16", I would hazard a guess that you might notice the difference, but nothing that a small crank on the ajdustable proporioning valve wouldn't solve. That said, if you were plumbing a car from scratch, I would run all 3/16" since less compliance is ultimately a good thing, even if you are splitting hairs.

Tobin
KORE3

[CreepinDeth]

Well , my new Raybestos premium NEW 79 Vette master arrived today.
So tomorrow I bleed it Tobin's method on the vise. Front angled down.....and this one came with plugs.
The 2 reman's came with tubes for the OTHER MC bleeding method. So that's a good start.

Originally Posted by CamaroAJ

just going to throw this out there since i know first hand this can happen. do you have the rotors bolted on or the wheels bolted on?

I did it both with the rear + front wheels off while bleeding AND did it with the wheels on.

Originally Posted by GregWeld

Just one last thought on this problem... I think we'd all ASSume that you know how to bleed brakes.... which is the furthest away first -- so passenger rear first - then driver rear - then passenger front - then driver front....

Yes sir. I did my research first. I followed that procedure EACH and EVERY time.

Originally Posted by GregWeld

I bench bleed the M/C on the bench rather than on the car. It's hard to get all the air out of the M/C sometimes. I use a large phillips screwdriver as the push tool... and that way I can really bottom the plunger. I have a set of lines I use that hook up to the outlets and return to the M/C reservoir and I can SEE that there is not a single bubble left before the install. I cap it -- and leave the lines on - get it bolted up - toweled up - and swap the lines over.

EXACTLY word for word how I did both my Vette masters.
Except I didn't plug them going from the vise to the install.
1st one went good on the 78 z28, front C5 brakes, rear stock drums, 1st time worked.
2nd one not so good on my 442, front stock disc, rear Baer disc, total TARFU mush pedal.....lol

One other thing that is a mistake I've found several times trying to help someone with "brake issues".... Not every M/C is the same line configuration - i.e., some M/C's run the front reservoir to the fronts and some run the front reservoir to the rear.

79 Vette masters are equal size reservoirs.
Several guys on Nastyz28 have used this with success this way too.
So I'm not guinea piggin it.

[Hydratech®]

If all else fails, you may have to get into port plug diagnostics. Even though this article is written for hydroboost applications, the basics still apply to a manual or vac assist brake package: http://hydratechbraking.com/Hydratech/braketech1.html

For instance, you could plug the ports at the rear axle flex hose, obtain a good pedal, THEN connect one rear caliper at a time, bleed it out and see what you've got. Since this axle was handled / transported, it strikes me that you may even be fighting an ever so slightly tweaked caliper mount bracket (that could've been thumped while wrestling the axle around).

[CreepinDeth]

Well it wasn't the master!!!!

So instead of taking the Repro Vette master off and plugging it and checking to see if it was bled properly,
I decided to just ASSUME it was bad, paint the new Raybestos one, plug it, bleed it, install it.

Then for the hell of it I threw the repro MC on the Vice and plugged it.
Bench bled it , plugged, with the screwdriver and ZERO bubbles came out and it locked up before 1/8".
I tried 15x and it's definitely bled properly. So now I really have no idea WTF it could be if it wasn't the master.

Tobin, you're saying air in the system, but I really bled the crap out of it several times. It's aggravating the ****
out of me. I'm wondering if I really do need a smaller bore MC but then other guys are running rear discs too with
this MC but it's possible the pistons are different. I"ve got power brakes, I thought it would be cake......nope!!!

Originally Posted by Hydratech

Since this axle was handled / transported, it strikes me that you may even be fighting an ever so slightly tweaked caliper mount bracket (that could've been thumped while wrestling the axle around).

IF that was a factor in the hydraulic system, I'd see some fluid leaking somewhere for the mushy pedal though no ??

I don't see any fluid leaking.

[Hydratech®]

Great! Now you can definitely rule out the MC as the culprit, and can continue moving your diagnostics further downstream in the brake system.

FYI - A bent caliper support bracket would be not readily visible to the untrained eye, and could cause the caliper to deflect crooked / cocked ever so slightly off plane in relation to the brake rotor. Then when you try to apply pressure to the caliper, you are "wasting" all of the fluid volume going to the caliper while the caliper straightens itself out by trying to clamp straight on the rotor. Take a close look at the calipers, pads, rotors to see if you spot anything ever so slightly "off" angle. Take a bright light and see if there is any air gap at say the front of the pads, yet no air gap at the rear of the pads (or on any other plane in relation to the rotor).

One glaring question - are the bleeder screws at the tops of the calipers? I just can't tell you how many times I have seen the calipers installed backwards (left caliper swapped with the right caliper, finding bleeder screws at the bottom instead of the top. You could run a tanker truck of fluid through the brake system and never get the air out if this is by chance the case.

Humor me - run up to the auto parts store and get some 3/16 brake line plugs in the brass section, then plug the rear axle flex hose where the steel brake lines go out to the calipers. Fill the MC up with fluid, leave the two ports open on the end of the brake flex hose, then wait for the fluid to start dripping steadily out of the ports on the rear axle brake flex hose (gravity bleeding discussed in our online article that I put a link up to in my previous post). Once the fluid is dripping steadily, then go ahead and pop the brake line port plugs into the flex hose and snug them up. If you then find a solid brake pedal, you have ruled out everything else in the vehicle's brake system except for the rear disc brakes (where this whole problem started). You could then connect one rear caliper at a time to the rear axle flex hose and bleed it out and see what happens. If everything goes bad again with say the driver's caliper hooked up (or vice a versa), then you can focus your attention to the specific area and then likely nail the problem.

Pneumatic testing: While potentially very messy and 'dangerous' to painted surfaces, you could also take a rubber tipped blow gun and actuate the brakes by hitting the brake lines (at the MC) with compressed air (use a large towel to catch any possible brake fluid). I know we are trying to rid the brake system of air, but push come to shove sometimes you do what you gotta do to figure out what in the heck is going on. With the compressed air, you can watch the brakes engage and release when you hit them with pressure, and might just spot something.

[CreepinDeth]

Originally Posted by Hydratech

One glaring question - are the bleeder screws at the tops of the calipers? I just can't tell you how many times I have seen the calipers installed backwards (left caliper swapped with the right caliper, finding bleeder screws at the bottom instead of the top. You could run a tanker truck of fluid through the brake system and never get the air out if this is by chance the case.

Are you serious ??
That's how these are......the bleeder screw is at the BOTTOM of each caliper on both sides.
This is the passenger side caliper, and right is front. The bleeder screw is at the very bottom.
My front stock disc brakes bleeder screws are at the very top.

Another member took this whole rear end off his 70 something Chevelle and it was functional.
So I'm baffled as to why that would all the sudden create problems......but not doubting it.

[CreepinDeth]

YOu know what ??? You're right!!!! EVERY SINGLE PIC I FIND......is the other way around.

http://www.mustangmonthly.com/howto/.../photo_23.html
http://www.mustangmonthly.com/techar.../photo_20.html


I have to go look up install instructions now to see how the hell that happened , and if I can switch them with minimal issues.
THANKS!!!!! I would have thought the Vacula would have sucked out the air though.....but I guess not.....worth a shot I guess.

http://www.buybrakes.com/store/BAER-4302002

[Hydratech®]

I suggest you swap the calipers and live happily ever after with the bleeders up top where they are supposed to be.

What the previous owner may have done to get it to work this way: While absolutely most certainly not the right way to do it, you *could* yank the calipers loose while leaving the brake lines connected, put a some appropriately sized hardwood inside of the pads to take the place of the rotors, orient the bleeder screws up top, and then bleed the air out the brake system. Once bled, you could then re-install the calipers and take the vehicle out for a test drive. If you feel that all is well at that point, then you could swap out the calipers right / left at any point, though I do suggest you get it done right now...

[CreepinDeth]

I got my work cut out for me today. I'm kinda a brake newb still....

I'm swapping them. Thanks again guys!!!! Muchos Grassy Ass

[GregWeld]

Pictures are worth a 1000 words my mother used to say....

In this case -- one picture might have saved a COUPLE THOUSAND!!

LOL