Weak Brakes/Too Much Pedal Travel

[SoCal_Cougar]

Hey all, first time checking out the new forum site, looks pretty nice, glad we made the swap to a more modern platform.

I’ve got an issue with my brakes, that I’ve been chasing since I got the car together, and I just can’t seem to fix. The brakes are weak, not weak enough to not stop the car, but I just feel like they must be able to be firmer than they are now.

I’ve got a mishmash of parts that do match up on the important aspects, such as the 69 brake booster and pedal which the PO swapped in, and then the 70 Master cylinder, lines, and distribution block, and in the front someone swapped to the 67 4 Piston disc brakes.

I’ve had topics before, every few months or so where I get to working on them again, and try to get any new info, so I’m back again hoping someone will chime in with something I’ve not heard.

I’ve bled the lines many times, with a Motive Power Bleeder kit, and bled entire bottles through each corner, and I get no bubbles after a few seconds, but I let it fill the whole bottle just to sure. I’ve adjusted the rear brake shoes out, made sure there are no leaks, and installed a new adjustable proportioning valve for the rear. Cleaned out my distribution block, and it wasn’t dirty inside at all, and the pin move fairly freely.

I bled it all again today (after putting a new flexible line from the block to the prop valve) and when bleeding the rear, I had very very slow flow, but then I adjusted the prop valve all the way in and then back out, and it sped up a ton, not sure why that would occur, but now at any position it flows freely and much faster, so it doesn’t seem to be a persistent issue.

The only part of the system I have not replaced is the rear rubber line, which I’ve heard can swell up inside and block flow. I figure that’ll be the next part to tackle, as it’s the only problematic part that I’ve not looked at.

Currently the brakes work, and I could drive the vehicle, but it feels like the first 80% of the pedal only engages maybe 10% of the brakes, and the last 20% of the pedal travel engages the rest, so I have to push the pedal most of the way to notice any actual stopping power. All the shoes/pads seem well adjusted, and the wheels spin freely with a bit of dragging on the shoes/pad. The only thing I could think is that the shoes are not matching the curve of the drum, and the top or bottom is hitting and stopping most of the actual shoe from contacting the inside of the drum.

I’m pretty much at a loss however, but finally have some time to work since the SoCal heatwave is over. Any advice would be appreciated, thanks!

[Hickey]

if all is done except the rubber line, do it… its cheap and easy to replace. When I pulled mine, i was surprised flimsy it was when I flexed it. I had a substantial, like change lanes when you put your foot on the brake, brake pull on my 91 Olds Wagon and it was a weak brake hose on the driver side that was causing that caliper not to have much force… easy fix or at least easy to rule out…

[xr7g428]

The push rod length may be a little short. Have you checked it?

[SoCal_Cougar]

Thats what I figure with the rubber line. I know of a shop near me that makes custom braided steel lines but im not sure if they can do that fitting on the end of the rubber line that connects to the 2 hard lines.

Last I posted about this we talked about the rod length, and I remember that the concensus was slightly under and inch, about .985. Ive had it both under and over an inch and noticed no major difference.

I do have a question however, and I assumed that I did it correctly, but when you set the rod length and install the master cylinder, there shouldnt be any “preload” on the piston correct? The way I determined the correct length was by adjusting the rod, feeling if it was compressing the piston at all, and once it was short enough to not compress the piston or only compress it maybe a tenth of an inch, I’d install it. This is how I arrived at the slightly under 1" mark before someone had even mentioned it.

I know that if the rod is too long, the piston can’t return all the way and you build pressure, but the only thing I could think of regarding rod length is that perhaps I assumed incorrectly and when I install the master, the rod should push the piston in slightly and have some “preload”.

The other option I considered was going to a larger bore MC but the one on WCCC that said it was 1&1/8" was actually only 1", and I can’t seem to find any that are larger

[Royce]

A larger bore size seems unnecessary. 1" should work fine, especially with a booster. The line assembly should be an available off the shelf part at any auto part store. I suspect you have trapped air somewhere. Maybe the front calipers are swapped side to side? Can you post a photo (extraordinarily easy on this new forum software)?

[SoCal_Cougar]

I figure a larger bore is unnecessary as well, but it would be a last resort. Once I get a chance I’ll post the calipers, however just for description, they’re the 67 4 piston calipers, both of them are on the front of the rotor, with the metal line and bleeder facing upward.

Ive thought that perhaps there is some air trapped somewhere, but just yesterday I bled a full bottle through each corner and got no bubbles past the first few drops. I’ve also bled with the 2 person method and had the same results. Perhaps a vacuum bleed or gravity bleed may be able to push it out if there is an air bubble trapped.

I’ve bled it on flat ground as well as on my driveway which is sloped, gotten the same results, not sure if it matters.

[xr7g428]

It’s just so easy to lengthen the rod and try it. It doesn’t have to be far off to get a noticeable lack of braking

[SoCal_Cougar]

I’ll try lengthening it today. I’ll take pictures as well of how long and try to see if it has any effect. I know that if I do extend it, the piston in the MC will be compressed even without pushing the pedal, since its already right on, hopefully it cause it to not release pressure.

[Royce]

More likely air is trapped and it will stay trapped because of an assembly error. This would yield clear fluid when you bleed - the air is trapped and no amount of bleeding will un - trap it.

Many folks use replica '67 4 piston calipers on later model vehicles - it’s a very common thing to have exactly the components you have and for everything to work perfectly.

[SoCal_Cougar]

How would I remedy that assembly error? Is there a sequence in which I’d have to connect and bleed the lines in order to get rid of a trapped air bubble if there were one?

[Royce]

It’s just a guess. A photo of the calipers will tell us.

[SoCal_Cougar]

Heres both the driver front and rear brakes. I adjusted the rear shoes out a few clicks, maybe 3 or 4. I’ll get to the booster rod in a bit.

[Royce]

Looks right. The rear shows need to be adjusted out until the drum refuses to rotate, then back off until the drum can be moved with difficulty.

[SoCal_Cougar]

Thats how I’ve done it, got it on there till it won’t move and then back 3 clicks. I’ve got video showing the rear shoes moving with the pedal, so there is fluid getting there and pushing it. Drove around the block, not much/any better from what I can tell. Working on the rod right now, I wish I didn’t have to remove these lines to get to it but that’s the way it goes. I see more bleeding in my future.

[SoCal_Cougar]

Just some preliminary observations on the adjustable rod. From the flat face of the booster where the studs come out, it is 1&1/16", which is 1&5/8" from the recessed hole in the booster where the master cylinder bore goes. The depth from the pison face to the end of the hold is 1&5/8" so currently it has no slack, no freeplay, as soon as the rod moves the piston also moves.

You can see the dark spots serve as an indicator of where the rod sits, there’s even a slight ring where the full extension of the rod is when the pedal is to the floor.

What I just noticed that may be odd, is that from the top of the pedal to the floor, I only have 1&1/8" rod travel, so it’s only compressing the piston that far in the MC. Not sure if thats enough, Im fairly certain when I bench bleed the MC I can compress much further.

[SoCal_Cougar]

Bench bled my master cylinder while it was out, and adjusted the rod longer, but unfortunately it did not help any.

I verified that the rod travel is fine, while I was bench bleeding I measured how deep I could compress the cylinder, and it was 1&1/8", which is exactly how much travel my rod has when I push the pedal to the floor.

I set the rod back to where it was, beforehand, since I didn’t want the piston to be compressed like it was when I lengthened the rod.

At this rate, the only things I can think of that I’ve not replaced/checked would be the rear soft line, and I’ve not checked if this MC has a residual pressure valve for the rear brakes. I got this MC from WCCC, and so I’d assume it has one, but I don’t even know if not having one would cause these issues.

Also, I assume this is normal, but I have about 3/8" pedal travel that does nothing, it’s definitely something within the booster. I know the reaction rubber disc is inside so it’s not that, I was just wondering if that was normal.