Trying to figure out brake problem with rear disc upgrade

[CreepinDeth]

Originally Posted by Apogee

Wow...I should buy some stock in whatever brand brake fluid you're using.

I've killed a gallon already......wasn't fun.


Okay, so I guess the '79 Vette MC is a single. I hope this new MC works out better.

Originally Posted by Apogee

As for the MC fittings, I'll usually bleed the MC fittings first by cracking them one at a time while under pressure just so I don't have to push those air bubbles all the way through the system, especially if it's already been filled.

So let me make sure I understand this.
You crack the 2 fittings while pressing the plunger in on the MC.....to get the air out.
Then before releasing the plunger you tighten the 2 plugs ???

....or did I misunderstand that ??

[Apogee]

Originally Posted by CreepinDeth

...So let me make sure I understand this.
You crack the 2 fittings while pressing the plunger in on the MC.....to get the air out.
Then before releasing the plunger you tighten the 2 plugs ???

....or did I misunderstand that ??

That's about right, except I just do one fitting at a time since I'm usually juggling a rag and trying not to get brake fluid everywhere. Have someone press and hold the pedal while you crack open and then close one fitting at a time and repeat until you're satisfied. I've found that 1-2 times per fitting seems to be plenty most installations.

Tobin
KORE3

[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.

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. The way I've found to check for this is while I'm bench bleeding it - I plunge slowly while looking for the fluid flow FIRST in which bowl of the reservoir - that reservoir should be to the fronts. If you do this a couple of times - you'll see what I'm talking about - you'll see fluid just a nano second before in one bowl or the other. That wouldn't give you a mushy pedal - I'm just adding this since you're using parts you may or may not be familiar with...

If you bench bled the M/C - and went thru a gallon of fluid - I say you've got a bad M/C and should probably just get a new one.

[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? it may sound like it shouldn't matter but this just happened to me the other day and then i remembered i didn't have the rotors bolted onto the hub. but i blead the system then had a soft pedal. bolted the wheels back on and have nice hard pedal. the rotor being able to move side to side even though its just a little bit is enough to push the pistons back and give you problems

[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....

[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.

[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.