Long pedal and grabby

[Sixt8Chrgr]

I have a 68 with power drum brakes all the way around the car.

I have what seems to be excessive pedal travel, and the brakes grab like crazy when I am going slow. I have adjusted the shoes out for the pedal travel with no change. I have no idea what to do about the grabby brakes at slow speed. I want to keep things original, but these brakes really suck. I have fluid in the reservoir. Any ideas on the long brake pedal travel?

Thanks,

Lawrence

[Belgium R/T -68]

If air in the system is your problem I'm the guy with the experience.

Per

[dpixel]

If the shoes are installed backwards they will be grabby.  Mine rear shoes were like that when I bought my car.  I believe the shorter shoe goes towards the front(in my rear brakes anyway).

[Musicman]

Long Pedal.... Sounds like you are missing the residual pressure valves.

[Sixt8Chrgr]

Long Pedal.... Sounds like you are missing the residual pressure valves.

Where do they go?

[b5blue]

Power brakes have an adjustable rod between the booster and master that needs to be set properly. Grabby brakes could be old or weak return springs and or not lubing the rub points on the backing plates. I've seen backing plates that actually have notches in those flats (where the shoe rubs) and in that case file them flat. 

[John_Kunkel]

I would first check the pushrod adjustment that b5blue mentioned, especially since there is excessive pedal travel, if it's adjusted too short the booster will hit the MC hard when the pedal is first depressed; the pushrod should just touch the pocket in the MC piston without actually moving the piston.

A faulty reaction valve in the booster can also cause grabby brakes.

[Musicman]

Where do they go?

If the master is original, then the valves are built into the front and rear brake outlet ports. If the master has been replaced with a rebuilt or aftermarket unit, you no longer have them. They would have to be purchased separately and installed in the brake tubing's, one for the front and one for the rear (drum brakes only).

Also check the push-rod length as B5 & John have suggested... Unbolt the master and measure precisely, the depth of the piston from the masters mounting surface. Now start the car and measure the rod length from the mounting face of the booster. Ideally, when assembled the tip of the rod should be as close to the piston as possible without actually contacting it. If it is to far away then you will have excess pedal travel. If it is to long, it will actually be engaging the piston and not allowing it to fully return.... each stroke of the pedal will preload the brakes more and more, and eventually they will begin to drag or lock up all together. Then you will have to wait for the excess pressure to bleed off before moving again, at which point the process will start all over again.

[Sixt8Chrgr]

The master and booster are original, but have been off the car.

It sounds like the rod is not adjusted properly. I will look into that.

Thanks,

Lawrence

[b5blue]

Mine was pulling, that was strut rod bushings (and 1 rod bent) and it had this trick of if you really put the brakes on hard and kept them on all the way to almost a stop the pedal stayed down till you stopped (!) I replaced the front wheel cylinders as they were seeping a tiny bit. That firmed them up reducing travel but I had to change out the M/C to really get things right. So now I've overhauled all 4 drums and M/C to new specks even though next year I'm changing to 11 3/4 disks. Stopping is more important than going!

[Sixt8Chrgr]

Today while at a stop light the pedal just sank to the floor. The brake system light came on. I pumped up the pedal and things were better but the brake system is still grabby and I think I have excessive pedal travel.
I think the first thing I have to do is confirm that all the air is out of the system and confirm there are no leaks. While doing this I can figure out of the pads are on backwards and then finally confirm the rod to the m/c is adjusted correct.
Does this sound reasonable?

[b5blue]

Yup! 

[Musicman]

Today while at a stop light the pedal just sank to the floor. The brake system light came on. I pumped up the pedal and things were better but the brake system is still grabby and I think I have excessive pedal travel.

It's beginning to sound like "trapped air" in the tubing's or the wheel cylinders. If the brake system light came on, that means that the hydraulic safety switch located in the distribution block, slid far enough to the front or rear to activate the warning lamp. The only way that can happen in a fluid system is by compressing air that has been trapped within the system, or by discharging brake fluid through a leak in the system. It is possible for wheel cylinders to ingest air over time if no residual pressure valves are present in the system.
Sounds like a good thorough inspection is in order, as well as a complete brake system fluid flush. Old fluid absorbs air as well, so start at the fluid reservoir and just keep flushing the brake lines with new fluid until the old crap is gone... and the air with it.

[Sixt8Chrgr]

Thank you.

[b5blue]

Be certain they are adjusted fairly snug also, long but firm peddle travel is a sign of this. In the past I had one wheel in the front that would fail to adjust proper and that was a sign to crank on it. Mushie peddle is air in the system.

[John_Kunkel]

If the problem was air in the system the problem wouldn't be intermittent, the pedal suddenly sinking to the floor is a master cylinder problem...the MC is bypassing internally. Time for a replacement.

[b5blue]

 

[Musicman]

If the problem was air in the system the problem wouldn't be intermittent, the pedal suddenly sinking to the floor is a master cylinder problem...the MC is bypassing internally. Time for a replacement.

Very true John, but on the other hand a bad master shouldn't be so intermittent either, and it shouldn't cause the warning light to actuate... Normally, fluid would have to be moving downstream of the distribution block for that to happen. Although it is possible for a really bad master to literally recirc and give the same indication. There may very well be more than one problem here, but you have to start somewhere... I'm simply suggesting that he start with a good system flush and a thorough visual inspection of all the components first. The master can be tested well enough with the car sitting still in the driveway.

[John_Kunkel]

I disagree, every MC that has bypassed on me started out as intermittent, I (wisely) replaced them before that totally failed.

The warning light illuminates because only one of the pistons in the MC is bypassing, as long as one piston still provides some pressure there will be an imbalance in the systems and that's what triggers the light.

[b5blue]

For 35 bucks put a new...not rebuilt M/C on to be safe. 

[Sixt8Chrgr]

For 35 bucks put a new...not rebuilt M/C on to be safe. 

I want to try and stay as original as possible. Where do you suggest that I buy one?

I know that I can get my original rebuilt at whitepost, but I dont have to spend that kind of money either.

Any suggestions?

[b5blue]

I got a new (not re-built Doorman) from rockauto on the cheap= 35.00=made in china, works but not "exactly correct" looking. It stiffened up my peddle that last little bit (my 4 wheel cylinders are all new also) I'm switching to 11 3/4 disk as soon as money permits so I had no issue with looks or longevity to any great degree and as my car was a 318/904/7 1/4 axle new and is now 440/727 /8 3/4 bastard exact correct could never apply anyway. If your staying with drums get a better M/C than I did or get yours re-done.

[Sixt8Chrgr]

NAPA has an original rebuilt for $30 with the core. I actually have not seen it in person just online. If in fact what they have in the store is what they show online I will probably go that route.

Thanks,

[b5blue]

Do not give them your core until your certain it's repaired. 

[Musicman]

Replacing the master can't hurt anything, and it may very well be the source of all your problems. Outside of a leak in the system, there are only a couple things that I know of that would cause a pedal to suddenly drop to the floor. One of them is a bad master, which is probably the case here as John suggests... another is trapped air in a wheel cylinder with bad seals, that will expel some of the air under pressure. Flushing the system will ensure that any trapped air gets removed from the system, and will be required if you change the master anyway. Don't forget to inspect the rest of the system and components while your there... and remember... the newly rebuilt master will not have the pressure valves built into it... none of them do. Also remember that you need to prime/bleed the master before use.

[Sixt8Chrgr]

I took the car to the shop today. Good thing I was taking it today because the brakes were really bad. When stopped the pedal would go to the floor about 3 out of 5 times. I had to pump up the pedal to get brakes.

What are the pressure valves?
Where are they?
What do I do with them?

Thanks,

Lawrence

[Musicman]

I took the car to the shop today. Good thing I was taking it today because the brakes were really bad. When stopped the pedal would go to the floor about 3 out of 5 times. I had to pump up the pedal to get brakes.

What are the pressure valves?
Where are they?
What do I do with them?

Thanks,

Lawrence

These 10 PSI valves are used in a drum brake system to prevent air from being ingested into the hydraulic system when you release the brake pedal. Typical wheel cylinder seals only seal when there is pressure behind them. Rapid release of the brake pedal creates a vacuum in the system which causes the seals to relax and air is ingested into the wheel cylinders. Maintaining 10 PSI in the system at all times prevents this. They also hold the brake shoes closer to the drum, maintaining a higher brake pedal, so you are not constantly pumping the brakes to stop. Older drum brake master cylinders had 10 PSI residual pressure valves installed internally behind the tubing seats, but in-line replacements can be purchased and installed "if necessary". Also, some newer style wheel cylinders have cup expander's which eliminate the need for the residual pressure valve.

Get a new master on there, flush everything real good, inspect everything, and then see what you get. You can always add the residual valves later if you feel you need a higher pedal.

Oh yeah... Here's a couple of shots of a typical drum brake assembly front & rear, just in case you don't have anything to go by already.

[Sixt8Chrgr]

Is a master cylinder rebuild kit an option or should I just put a new or rebuilt mc on the car?

Thanks,

Lawrence

[Musicman]

Just put another one on there... new is better. Many times rebuilt units are not all that they should be.

[Sixt8Chrgr]

Just put another one on there... new is better. Many times rebuilt units are not all that they should be.

Ok I will go with a new one.

[Sixt8Chrgr]

Musicman if I need to install in line pressure residual valves, are they installed inside or outside the master cylinder? Can you give me some details about this if in the event my wheel cylinders don't have the internal valves you spoke about earlier?

I want to ensure that I have a hard pedal at the top of the pedal travel. I need some insight on this please.

Thanks,

Lawrence

[Musicman]

The 10 psi aftermarket valves are installed in the brake line tubings themselves. You can install them somewhere in the lines coming out of the master if you like, one for the front drums, and one for the rear drums. They can be purchased at any number places : MP Brakes, Pirate Jacks, SSBC, etc...

[Sixt8Chrgr]

OK thanks

[Sixt8Chrgr]

Musicman thank you for all the help on the brake rebuild in my 68.

This is what we found:

Leaking master cylinder at the back of the unit
Installed a remanufactured unit from Auto zone because they had no new ones in stock. The remanufactured unit is working fine and was only $31 with no core replacement. I will probably rebuild the original and put it back on when I restore the car.
The rear brakes were not working at all. The rubber hose on the rear end had collapsed and no fluid was going to the rear brakes at all! Once we got the fluid to the rear brakes the wheel cylinders leaked so we put new wheel cylinders on the rear. The front wheel cylinders and the rear ones were very new looking but I guess the rear ones failed because they did not have any fluid in them? Who knows, but I now have 4 new or like new wheel cylinders.
The fluid was old and nasty looking when we pulled it through the system. We flushed the entire system with good fluid during the rebuild.
The brakes were horribly out of adjustment. We fixed all that and now my brakes work perfectly. The car is a pleasure to drive now. The power drum brakes are not that bad actually.
Again thanks for all the help.

Oh we did not have to put in the 10 psi valves.

Lawrence

[b5blue]

There you go! (or stop going)  The drums work well as long as you don't expect 100mph stopping power.

[Musicman]

Laurence

It's good to hear that you got everything going again. Congratulations on a job well done

Mike

[Sixt8Chrgr]

Thanks b5Blue and Musicman