Master Cylinder bleeding - rear port doesn't push fluid immediately?

[ULTM8Z]

So I bought a 67-76 Corvette master cylinder with 1" bore. While bench bleeding, I'm noticing the rear side of the MC doesn't start pushing fluid right away. The front pushes fluid immediately..

On the rears, I don't start getting fluid movement until about half way along the piston stroke.

I then put my old MC on and it's doing the same thing. WTF? Don't know why I didn't notice it before, but I think that explains my late pedal? If I don't get fluid going until mid-stroke on the piston, that would explain why the pedal seems to always engage late.

The front brakes have been hooked up to the larger rear reservoir (it's one of those cast iron MC's with the large reservoir in the rear and small one up front).

Shouldn't both ports push fluid at the same time upon pressing on the piston?

[egoman]

Its been years since I had one of these apart but let me have a go at this.
The piston by its design has one of its sections that is on a spring. Otherwise when one brake engaged fully the other wouldn't work.(The system would go into hydraulic lock on which ever set of pads hit the rotor first. Because fluids are non compressible the pedal could move no further and the other circuit would never work!) What you describe would be explained by this very design. While I am not sure, when its a bled and on the car this should go away. But it is also the reason that dual cylinders are used for HP braking systems.

[ULTM8Z]

Looking at all these images on Summit and Jegs for pre-plumbed power disc brake kits (4 wheel disc) it seems the front MC port always goes to the front of the proportioning valve regardless of the MC type.

[Skip Fix]

81 TA 4 WD MC front reservoir is for the rear brakes.

[ULTM8Z]

I actually ditched that one... no way the 7/8 bore was going to do what I want

[Skip Fix]

Generally largest reservoir is for the front brakes as they generally have larger pistons and need more fluid volume reserve.

TA MC 1 1/8

[ULTM8Z]

Even on MC's with different reservoir sizes, I'm seeing the front (smaller) reservoir plumbed to the front.

I can't see any way where the caliper piston movement would empty out a reservoir (even the smaller one).

Any event, the Corvette MC has 1" bore which I'm going to try. It's reservoirs are about the same size anyway...

[Skip Fix]

Probably not empty it unless as the pads wear and pistons go out more and you do not check level. But why would the smaller reservoir go to the front.

[ULTM8Z]

I'm not sure why... which is why instinctively hooked the rear reservoir to the front brakes and never gave it a second thought.

Any rate it'll be interesting to see how the system behaves now. Won't know until this weekend.

But if the large reservoir is supposed to go to the front discs, why would Baer supply a MC that's essentially backwards?

[Skip Fix]

Same reason GM did on the 81 cars and why teh Strange/Mopar MC recs the largest volume on the back port. A quote from Strange.
"The outlet port nearest the pushrod supplies the most fluid volume and should be plumbed to the calipers requiring the most volume. Typically a drag race only car will have the largest calipers on the rear, while a street driven car will have the largest calipers on the front. Attach lines accordingly."

[ULTM8Z]

That only seems to make sense if the rear port pushes fluid immediately. Since the front discs do the vast majority of the stopping effort, seems like you wouldn't want them hooked up to a port with a delayed reaction (even if that port had the larger reservoir).

Which again, confuses me about Baer's selection of this particular MC to put in the kit. Seems like they would have supplied one with the larger reservoir on the front (if reservoir size is indeed a concern).

It's been a long time since I initially installed the kit and I don't have the instructions anymore (nor do I see it on their website). So I don't know what they said to do at the time. It could be that they said to hook the front brakes up to the front (smaller reservoir) and I missed it. Who knows...

[Skip Fix]

https://www.pro-touring.com/threads/127617-Critique-My-Brake-Setup

Current thread here- look at the picture of the S-10 MC -front port going to rear brakes.

[ULTM8Z]

I'm not sure the inner workings of a S10 MC. Everything I was looking earlier were the old iron MCs.

But based on how my MC is behaving it makes sense that the front port would be for front brakes. You wouldn't want the brakes doing the majority of the work coming in so late...

[Skip Fix]

I looked at the link picture on NastyZ28 and the front reservoir looks so small I would still bet it is not. Call Baer? Still sounds like the MC is not quite right coming in late.
http://baer.com/GM-Cast-Iron-.9375-Bore-Master-Cylinder-6150012.html

[ULTM8Z]

Yeah, I called them the other day and they were adamant that the front reservoir goes to the front brakes, and they are evidently aware of the "delayed reaction" of the rear port. In fact they still sell this thing as compatible with their brake systems..

http://baer.com/GM-Cast-Iron-.9375-Bore-Master-Cylinder-6150012.html

Any rate, I bought the 1" bore 67-76 Vette MC (4-wheel disc), as well as a new 11" booster and 4-wheel disc proportioning valve.

So aside from eliminating the delayed reaction on the front brakes, I can see which bore sizes gives me the pedal feel I want.

BTW, I also replaced the rubber hose in the rear out of abundance of caution (though the old one still seemed to be in good shape when I pulled it off despite being on the car since the mid-90's... I had a lot of heat shielding wrapped on it to keep exhaust heat from getting to it). The front lines are braided stainless ones from the Baer kit.

So essentially now I have clean sheet of paper I'm starting. Now with known "standard" parts for everything, hopefully I can get this thing straightened out.

[Skip Fix]

Always good to start with new stuff. I have had a bad "new" Vette MC also, it would just not bleed.

[AMC Racer]

A tandem master essentially has 1 piston connected to the pushrod and 1 piston floating, typically with a 2:1 volume ratio (one piston has an effective stroke twice the other, higher volume goes to front brakes). The secondary floating piston relies on pressure in the primary circuit for it to stroke.

When bleeding, you essentially have a leak in the system, so response will depend on bias of the springs and travel (if either primary or secondary system fails, i.e. has big leak, then pistons must travel further until they mechanically engage). Hook it up to your system so you have no leaks and if properly bled, you shouldn't have much delay in response (other than crossing the port that allows fluid in from the reservoir and initial caliper piston movement).

Note some masters have front the higher volume port and some the rear, and some have 1:1 ratios so doesn't matter. Need to check and connect to the correct ports. Reservoirs are sized so that people who never check brake fluid don't run low before pads wear out.

[ULTM8Z]

Wow, thanks for the detailed explanation. So you're saying this delayed reaction is only apparent during bench bleeding, but that it disappears when on the vehicle. Further, that the delayed response I'm getting is more a function of the volume difference between the two pistons.

Since on both my master cylinders that I currently have on hand (the 15/16" and the 1") both exhibit the delay during bench bleeding, I can infer that the volume difference is biased to the front port and thus front port should go to front brakes, no?

[AMC Racer]

Yes, delayed reaction is due to an effective open circuit, so not building any pressure. If you measure total oil volume out of each port for full stroke, the one with the higher volume goes to the calipers that take more volume, which is front on most vehicles. If you connect the low volume port to the high volume calipers, may not have enough volume and the mater cylinder piston may bottom out against the mechanical stop before pads are tight to the rotor, effectively giving you only one circuit working (same as if you had a leak or not bled).

Once bled, if you plug one port of the master then the other, should see near instant response out of the open port and clearly see the volume difference if it's not a 1:1 master. If you plug both ports, should have very little movement of the pushrod before it gets hard if it's bled correctly - just amount of movement to where piston seals pass the supply ports from the reservoir (often referred to as "travel to cut-off").

Hope this helps.

[Skip Fix]

Also most GM MC have the 9/16 larger threads for the rear brakes.

[ULTM8Z]

So I ran a test just now...

On my 15/16" MC (same as below) I measured the output of the MC on front and rear ports after one piston stroke. Both come up with ~5g of fluid output. I guess I would have expected closer to 10 on of them of the other was 5, (if in fact there's a 2:1 volume ratio).

I'm so confused now on this damn thing... nothing makes sense..

- 9/16" port is on the front reservoir - so implies front port goes to rear brakes

- Every picture I see with pre-assembled MC/booster kits with this kind of MC has the front port on MC going to the front on prop valve. Implies front port goes to front brakes

- I keep reading the larger reservoir goes to the rear brakes (due to larger movement of drum brake shoes). But this MC has the 9/16" port on the small reservoir up front.

- fluid volume output seems to be the same on both ports (even if there's supposed to be a 2:1)


I'd call Baer, but frankly I'm not sure I trust what they're telling me anymore...


https://static1.pt-content.com/images/pt/2018/02/Master01_Up_Up3QTRturn-1.jpg

[Skip Fix]

Rear drums cylinders have a lot less volume than a front disc caliper piston(maybe 1" diameter), why the rear on a disc drum and even disc/disc(as rear calipers have smaller pistons) have a smaller reservoirs for the rears.

Most stock second gen F bodies have a 2 15/16" front caliper piston, and the 79-81 TAs had a 2.5" rear caliper piston.

When you look at some of the specs of original 60 MC some have different % volume front to rear (60/40) than other applications do. The engineers took lots of variable in their design that we don't when we put an aftermarket or racing system together. Heck lots of road race and circle track cars use separate MC front and rear with a balance bar so they can fine tune front to rear piston sizes as well.

[ULTM8Z]

I found this thread on a search...

https://www.pro-touring.com/threads/10037-Help-with-Baer-brakes?highlight=master+cylinder+port

So maybe the rear port really is for the front brakes on this MC...

[ULTM8Z]

Ok, I think I'm becoming convinced that the front port is for the rear brakes on my original Baer MC...

I did a search online for images of "Baer master cylinder" and came across these photos clearly showing adjustable proportioning valves hooked up to the front port (obviously for the rear brakes)...


https://static1.pt-content.com/images/pt/2018/02/116_0305_baer_15_z-1.jpg

https://static1.pt-content.com/images/pt/2018/02/valve-1.jpg

And this one is from a Hot Rod magazine article about installing Baer brakes on a 1st gen Camaro...

https://static1.pt-content.com/images/pt/2018/02/116_0307_baer2_16_z-1.jpg

[AMC Racer]

Some tandem masters have a small hole drilled in the wall between front & rear reservoir to equalise fluid level in the reservoirs as pads wear. The hole is high enough to maintain a deep enough level so that slosh doesn't uncover the ports supplying the pistons under g-loads (especially braking load) and to ensure if there's a leak draining one side, other circuit still works.

Look at where the holes are in the bottom of the reservoirs that supply fluid to the pistons. Tip the master forward to see where both remain covered. May help explain some of the difference in sizing.

If you are getting same volume out of both ports under full stroke with a fully bled master and not allowing air back into the ports during retract (could verify with e.g. 10 full strokes to get a more accurate measure) then it's 1:1 and doesn't really matter which port you connect to if volume is the same.

Note some masters also have differential bores between primary and secondary, to give differential primary and secondary circuit pressures ... but not common.

IIRC my stock master also had the larger reservoir on the rear and rear went to front brakes and front went to rear brakes. Had to swap lines when I fit a Wilwood master.

[ULTM8Z]

Thanks. I actually solved my overall braking issue tonight... Turns out the original way I had it plumbed (front port to rear brakes and vise versa) was correct. A few other factors may have come into play to solve the overall issue, but suffice it to say, everything is working properly now.

So yeah, I guess to close this thread out as well (in case anyone in the future does a search with a similar issue)... here's the link to the resolution (post #30 in this link).


https://www.pro-touring.com/threads/127387-Improving-pedal-feel/page2?p=1257225#post1257225