Advice Please: 308 master cylinder/booster problem..

Jim,
Exactly--I thought about this, but that wouldn't account for the pedal bottoming out.

Gotta be air somewhere.

Wish I had more time to work on my passion--work has kept me busy the past couple days. I'll resume tomorrow and post findings!

We'll get to the bottom of this together!

As usual, thanks for the thoughts. Always appreciated.

 

OK, somebody explain THIS to me:

Again, I tested my new MC using 2 short brake pipes w/ clear tubes, dumping into a jar. Let it ran under MOTIVE and pedal pressure for 5-10 minutes. Again, no bubbles, not a one. This MUST mean that the MC is NOT the source of bubbles.....

So, I attached a longer brake pipe directly from the front line outlet on the MC to the left front Goodrich SS line at the bracket. This in effect bypasses 2 lines--from MC to junction block, and from junction block to the Goodrich SS line.

Guess what? Same symptoms re-appear!!! Endless bubbles!!

What the heck?

I can ONLY conclude that the caliper or SS line is somehow at fault. My little experiment proved that the 2 bypassed lines and junction block aren't to blame..

Seems impossible that all this air is coming from the caliper. Can't be that much volume in there. It's non-stop.

Brake pedal still bottoms out when I attach MC to caliper, does NOT when I
use the 2 short pipes, plugged.

Anybody care to take a guess? At least I'm narrowing the possibilities..

Thanks,
Greg

 

Milton,
I didn't need to. I tested it as described above. (posts #81, 103). There is no air in the MC.

I'm sure of it.

The problem lies downstream.

I'm going to back bleed from the calipers tomorrow and see if I have bubbles in the other direction..

Readers: please don't miss post # 103....

Thanks,
Greg

 

Yep, those are the only possibilities left.

The connections are very tight, and they were functioning fine before I got into this. Of course, I've checked them 1000 times, and they are dry and tight.

Must be a massive air bubble in the caliper(s). I'm going to back bleed today and see what happens. When I first installed the new dry MC it must have pumped a massive air bubble down the line and it (they) lodged in the
caliper(s).

Thanks,
Greg

 

OK, I back bled one caliper, and it seemed to help, but not sure. I also wrapped all bleed screws in Teflon tape. That might be what stopped the bubbles. After this I was able to get all bubbles out of the system. I went around the car 5X to make sure.

BUT, the brake pedal was still long. All the way to the floor in fact.

I lowered the car, backed up and dowm my driveway, and there are almost no brakes.

At this point I have to conclude that the MC must have the wrong piston depth, or whatever you call it. Can't figure out why this would be, because all air is out. I used both the MOTIVE and my neighbor pressing on the pedal.

I'm going to re-install (again) the OEM MC and see what happens.

Any ideas?

Greg

 

Uh, not sure. The booster rod telescopes out, into the MC piston.
That rod cannot be adjusted, I don't think, can it?

The MC piston itself doesn't seem to be adjustable. How do you tell?

This would explain my problem, wouldn't it? Not enough piston movement?

The brake pedal bottoms out and barely stops the car. I wouldn't dare drive over 20MPH...

???

Greg

 

Because the booster rod is fully-extended?

What if the MC piston needs to be adjusted to properly interface w/ the booster rod? Is this even possible?

I'm just exploring all possibilities...

Thanks,
Greg

 

Well, the MC has two outlets, front and rear. Single master cylinder for both front and rear.

Both lines feed into a junction block, and split off from there. The front line splits into 2, lines, and go directly to the calipers, and the rear line splits into the proporioning valve, then heads rearward, where it meets a T-block and splits to each rear wheel.

One reservoir, w/ one fluid compartment.

So, you suggest plugging one or the other MC line outlet to see if my pedal won't bottom out?

I have a short metric-thread brake pipe I've been using to test the MC, but it's open on both ends. I've tried to find closed fittings to plug it, can't find any. Does anybody know the thread pitch? Maybe I can find a bolt to match.

Also, can somebody PLEASE explain the interaction between booster and master cylinder to me?

Seems straightforward enough--pedal actuates booster, which in turn extends a telescoping rod to act upon the MS piston. The booster rod IS supposed to telescope in and out, right?

Thanks for hanging in there, guys,
Greg

 

Andy,
The Wilwoods do indeed have the bleed screw at the top.

Steve,
You're right, I'm about to go blind from this!! I know for a fact that my Wilwoods are fine. No need to yank them off--they worked fine before.
They are air-free and full of fluid.

Spiderseeker,
Yep, I've knocked the calipers, MC, junction block and proportioning valve with a rubber mallet many times throughout this. See below for an explanation of my "endless bubbles" Jim had it right earlier.....

After thinking about this tonight, here's where I think I am:

All the "endless bubbles" I reported were, I think, air getting in the bleed valves at the seat as fluid rushed out. It made no sense that it was "new air", because the MC tested air-free. Here's how I solved that mystery: I wrapped Teflon tape around all 4 bleed screws, and the air bubbles ceased! I tapped the calipers with a rubber mallet, no new air.
I bled all around today, 5X, no air at all. YET, my brake pedal continues to bottom out, and I have virtually no braking power.

So, I can only conclude that either:
1) my master cylinder piston is the wrong spec. It does bolt up perfectly, but it is a bit shorter. The piston action is the same as the OE MC. I checked that when I had them both on the bench.
OR
2) the booster rod is somehow not extending far enough into the MC piston.

Can anybody confirm for me that the booster rod does indeed telescope in and out? I just wonder that maybe I don't have it set right--maybe it needs to be set so that it's rigid.... This would explain the long pedal and no braking power.

Concerning my earlier statement about the OEM MC exhibiting the same symptoms, well, now that I think I've solved my "air" mystery, I need to re-install it and see how my brake pedal feels. It's been so long ago I forgot my findings at that time!

 

Thanks Pap. So, the booster rod does indeed telescope? It's not supposed to be rigid? I wonder if my booster rod is not pushing the MC piston far enough in...

Thanks,
Greg

Damn, this thread is now 120 posts long... Sorry

 

Paul,
Good idea, I'll try that. Not sure what you mean by "too large of calipers".
They are larger than stock, but my OEM MC worked OK w/ them, but that's why I'm doing this whole thing--to match a bit larger MC to my Wilwoods.

Pap,
The new MC is 25 mm versus 23 mm for the OEM. The case itself is the same circumference, but measuring the front seal opening, it's just a bit larger--the bore is slightly larger.

The reservoir holes and mount holes are identical to OEM.

It's a bit shorter than the OEM MC. This may or may not be a factor--don't know if that last 1/2 " of the OEM MC is critical to piston sweep.

Does the 308 series from 1976-85 use the same master cylinder/booster? This would be good to know--maybe I have the wrong master cylinder. It was sold to me as a "308 master cylinder", no year specified.

Thanks,
Greg

 

I just read an old 308 brake booster thread post where Philip Airey said that the booster rod clearance/slack had to be re-set when you R & R the master cylinder.

Does anybody here know anything about that? That could be my problem!!

Thanks,
Greg