Low braking phenomena

[Phred]

28 phaeton, mechanical brakes, one piece not multi piece service brake cross bar, brake rods and linkages rebuilt/refurbished and well adjusted. They can lock up straight and true at speed if needed. 99.99% of the time all works as expected. Once in a great while, the brake pedal will only travel “so far” and only allow 50% braking. No matter how hard the pedal is pushed, the pedal does not seem to move any more and braking is not increased. It is not a brake fade situation as the pedal does not go to the floor, Kind of unnerving.

My first reaction was to “pump” the pedal and after the first release of the brakes all braking returned to normal. In the subsequent few instances of the phenomena, releasing and reapplying the pedal corrects the situation first time every time.

I have visually watched the movements of all the rods and linkages and do not see anything even close to binding or any restrictions. Similarity, I do not see anything binding or twisted when I remove the drums.

Your collective wisdom would be appreciated.
Peace
Phred

[Bill G]

Sounds like something in the linkage is catching on something occasionally. Maybe the pedal shaft or the pedal to cross shaft linkage. Check for tight clearances.

[J Franklin]

Interesting, there must be something binding. Is there an axillary return spring?

[Phred]

Yes, there is a aux pedal return spring, driven nearly every day (1000 miles) since this April, was greased well in April. The phenomena probably once in every 1000+ brake applications(?). It has not occurred on the first application of brakes (after setting for a day or two).

I cannot do anything to make the phenomena happen at will.

Interesting...

Phred

[bavArian]

I'd disconnect the front or rear axle brake linkage first. If nothing changes with one axle disconnected, you can assume that the disconnected axle is fine. If you can isolate it to one axle, reconnect the good axle and disconnect only one wheel on the bad axle.



If nothing changes when disconnecting the axles, the error must be "further upstream".


Without having had such a problem, I'd guess that (like mentioned before) there is a tight clearance somewhere that likes to bind when the brake hasn't been used for some time. Maybe old grease?

[Jim/GA]

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Look very carefully at the front actuating wedges, the piece that the pin pushes down on that is bolted to the backing plate. Are both of these stock design? Or do you have improved floater design? I think one is jamming if it gets a little crooked or cocked on the bolt. Look for signs of metal jamming, burrs, etc.

[jb-ob]

Get Out & Under.....Check to see the service Brake Cross Shaft has full clear rotation not hitting the tranny, u-joint area.

[bobbader]

I think my friend JB-OB is on to something here. Clearance of service brake crosshaft to U-joint housing might be compromised. Also, kudos for the 1000 + miles recently driven and complimentary grease job. The next question might be ............... what, if any, maintenance, repair, inspection of the complete brake system has been done recently. Give us some history. How long have you owned the car? What, (again) if anything has been done to the brake system in the car's recent history?

Next question ................What might be moving when you "pump the pedal" to alleviate the problem? Whatever is moving in that scenario is reversing some movement that has caused the lack of pedal movement to begin with. Find what moves when you pump the pedal and you'll find what caused it to jamb the pedal in the first place. This might take some keen investigating. Next time the pedal jambs, stop the car however you might without pumping the pedal, turn the motor off, remove the floorboards, get yourself down in a position where you can slowly pump the pedal by hand and watch any moving parts in the nearby proximity to determine when and what in the pumping process releases the "jamb". This may involve a trial and error multiple time approach and may need you to enlist a helper to "slowly" hand pump the pedal while you study each moveable part in the system. As JB says ........ "time to get out and under". Otherwise, think of the consequences if the next time you're going down the road and your "pumping the pedal" might not provide the desired "fix".

Good luck and keep putting those miles on. Bob Bader

[Floats]

Hi Phred,
My 2c
Put a little side load on the pedal in order to attempt replicating the interference.
Regards
Chris
Cape Town

[Jack Shaft]

Dont want the one in a thousand to happen when you need the brakes.Do one wheel at a time disassemble till youve checked everything.Find the root cause and fix it.No shortcuts,check it all.

[updraught]

Brake pins binding in the king pin?

[Phred]

Thanks, pedal side load and brake pin/king pin interference are things I had not thought of.
Will recheck all again because sometimes you can look but not see...

FWIW, "pumping" the brakes is probably a misnomer. The phenomena goes away with the first release of the pedal and all works normal after the next brake pedal application.

Phred

[larrys40]

Phred

Not sure if you have located the trouble yet but as hinted above
There are several key things to check out I would look at first.
I’ve done much A brake work and comprehensive restores and have seen a wide variety of issues.
1. Are the engine mounts worn and causing the rear of the transmission to run on the brake cross shaft? If so that could cause pedal issues and is common on worm out cars .

2 if the front backing plates are lose he brake operating pin can bind in the kingpin causing issues . Tighten backing plate bolts very tightly and make sure the pins are not binding by removing them and inspecting.

3 make sure no brake pedal or rod from pedal to respective backing plate is binding or has issue.

Please report back

Larry shepard

[1931 flamingo]

Where is the batt cable from the batt to the starterrun?? Could it be interfering??
Paul in CT